For their serial arrangement, see p. 337. Korschelt and Heider state that they would be inclined to homologize the salivary glands of insects with those glands of myriopods opening into the mouth-cavity, were it not that these glands in myriopods opening into the mouth are in reality transformed nephridia originating from the mesoderm, while the salivary glands of insects are clearly ectodermal structures. We must, therefore, they add, leave to later researches the question of the homology of these organs, also of their relations to the similar glands of Peripatus.

The urinary tubes.—These excretory vessels arise as paired evaginations of the hind intestine or proctodæum. They are ectodermal structures arising as lateral diverticula of the intestinal cavity (Fig. 546). Figure 547 represents their mode of origin at the anterior end of the proctodæum of a locust. It will be seen that there are 10 primary tubes. There are 150 such tubes in locusts, or 10 groups of 15 each. The 15 secondary tubes probably arise from the primary ones in the manner described by Hatschek for Lepidoptera (see his Taf. III, Fig. 7).

While the Malpighian tubes usually first arise as diverticula of the proctodæum, in the Hymenoptera (Apis and Chalicodoma) they appear, even before the completion of the proctodæum, as invaginations of the ectoderm which at first open out on the outer surface of the primitive band. They seem, then, in some degree, to be similar to the tracheal rudiments, which perhaps is the reason why they have been homologized with them, a view which we do not share, and in which Carrière does not concur. They afterwards pass, with the growing proctodæum, into the interior of the embryo. (Korschelt and Heider.)

The heart.—The dorsal vessel is first indicated, according to Korotneff, by a long string or row of cells (cardioblasts), which on each side border the mesodermal layer of the primitive band (Figs. 544, h, 548, h). In the advancing growth of the primitive band around the yolk, this rudiment steadily passes up more towards the dorsal side. It is in connection with the wall of the primitive segment (Figs. 544 and 548), and represents the point at which the dorsal wall of the cœlom-sac passes into the lateral wall. According to Korotneff, the cardioblasts arise directly through a migration out from the wall of the primitive segment.

In Gryllotalpa the formation of the dorsal organ, which, as Korotneff states, is in this insect nothing else than a stopper which fills up the dorsal gap of the body-wall of the embryo, is effected by the rupture of the embryonal membranes. The serosa is drawn together to form a thick plate (Fig. 523, A, rp), and the much degenerated amnion-folds (am) which are laterally attached to it have moved from the edges of the primitive streak (*x-*y) far towards the dorsal side (see Fig. 539, C, which represents a similar stage). The distance between the rudiment of the amnion-fold and the lateral edge of the primitive band (*x, *y) is occupied by an epithelial lamella (l), in which we recognize the earlier amnion. This lamella does not lie directly on the yolk, but is separated from it by a spacious blood-lacuna (A, bs), in which can be seen numerous blood-corpuscles which have migrated in from the mesoderm of the primitive band. The cardioblasts which have arisen from the wall of the primitive segment (us) are on each side arranged into the form of a furrow (gr), which bounds the blood sinus below.

By the continuous growth of the primitive band around the yolk, after the resulting invagination and degeneration of the dorsal plate, the two blood-lacunæ unite together on the dorsal side into a single one (B, bs). These constitute the first cavity of the heart. The vascular furrows (gr) come in contact with each other and grow together, and the wall of the heart is thus formed. Ayers states that in Œcanthus the heart is formed in the head region only after the yolk-sac has passed entirely within the body. The venous ostia arise by two paired invaginations of the lateral walls, forming a split at their bottom.

The rudiment of the heart stands, as we have seen, in intimate union with the primitive segments. Out of the lateral walls of these segments, after giving off the elements of the somatic mesoderm, arises an epithelial plate which becomes the rudiment of the pericardial septum or dorsal diaphragm (Figs. 523, A-C, dd, 544–545, ps). As soon as the two halves of the rudiments of the heart have united with each other in the dorsal middle line, the two halves of the pericardial septum unite with each other and form the wall to the pericardial cavity and shut it off from the rest of the body-cavity. For a long time the pericardial septum remains in union with the wall of the heart. Afterwards, however, it separates from it (Fig. 523, C, dd). (Korschelt and Heider.)

The statements of other authors (Ayers, Grassi, Patten, Tichomeroff, Carrière, Heider, Heymons, etc.) as to the mode of origin of the heart in insects of other orders are all similar to the type described in Gryllotalpa. The difference consists mostly in the fact that the two large blood-lacunæ are wanting or only exist to a slight extent. It results that the rudiment of the cavity of the heart in the earlier stages is of slight extent and often scarcely recognizable.

In Œcanthus (Ayers) and in Gryllotalpa, the hinder section of the heart is the first to develop, the development advancing from behind forward.

The blood-corpuscles.—Blood-cells are said by Korotneff to be, in Gryllotalpa, at an early period present almost everywhere between the yolk and mesoderm; they are derived, as he states, from the cells of the somatic mesoderm layer, which has lost its connection with the other parts of the mesoderm, and fall into the body-cavity. Ayers states that the blood-corpuscles arise from serosa nuclei which have passed into the body-cavity, where they become more vesicular, and ultimately all of the nuclear substance goes to form from one to three spherical bodies, which are surrounded by the common membrane.

“These bodies are blood-corpuscles and are free nucleoli immediately on the rupturing of the vesicle which surrounds them.” (Ayers, Pl. 22, Figs. 1, 3, p. 250.) More recently, Schaeffer has observed in caterpillars certain cell-complexes associated with the fat-body which he has called blood-forming masses.

Musculature, connective tissue, fat-body.—The muscles of various parts of the body, as well as the connective tissue, arise by histological differentiation from the somatic layer of the mesoderm (Fig. 523, so). The fat-body originates from the same source, as shown by the researches of Kowalevsky, Grassi, and of Carrière. In Hydrophilus a dorsal band of the fat-body passes over the digestive canal arising by direct transformation of the wall of the cœlom-sacs. But also the other portions of the fat-body, as the fat-body lobes accompanying the tracheal system, are of undoubted mesodermal origin. Heymons’ observations on the cockroach (Phyllodromia) agree with the foregoing view. In this insect at a very early period certain cells in the wall of the cœlom-sacs undergo a change, and may be recognized as the rudiments of what are afterwards fat-body tissues (Fig. 540, B and C, f).

The reproductive organs.—Our knowledge of the mode of development of the genital organs is in a less satisfactory state than that of the other organs. It is now known that the rudiments of the sexual glands belong to the mesoderm, and are developed from the wall of the cœlom-sacs. In the cockroach (Phyllodromia), the most generalized of the winged insects, as Heymons has shown, in the earlier stages of the embryo separate genital cells are already distinguished by their histologically different characters from the other mesodermal cells. The genital cells are larger and show a feebly stained nucleus with a clear nucleolus. These genital cells, which are transformed normal mesodermal cells, lie originally within the mesoderm layer or on the surface of this layer turned towards the yolk, on the edge of the segments. After the complete formation of the cœlom-sacs we find them (Fig. 549, gz) in the dissepiments which separate the successive cœlom-sacs from one another. Here new genital cells are constantly formed through the transformation of mesoderm cells. The development of the genital cells takes place in the 2d to the 7th abdominal segments.

Afterwards the genital cells pass into the interior of the cœlom-sacs, and soon pass to the dorsal wall of the same (Fig. 540, A, gz) and enter between the cells of this wall. The cœlom-sacs (c) show in cross-section in this stage a triangular outline, so that we can distinguish a dorsal, lateral, and median wall. The dorsal wall lies next to the surface of the yolk, and afterwards gives rise by separation or splitting to the splanchnic mesoderm (Fig. 544, sp), while from its remains the terminal thread-plate (ef) originates. The lateral wall, which is turned towards the ectoderm of the primitive band, is intimately concerned in the formation of the somatic layer (Fig. 540, C, so) of the mesoderm. Out of what remains arises the pericardial septum (Fig. 544, ps).

When the genital cells have entered into the dorsal wall of the primitive segments, they are already so numerous that they form a continuous series extending from before backward. The genital rudiment consists, then, of a string of cells lying on each side in the dorsal wall of the primitive segments, which extend from the 2d to the 7th abdominal segments. In the formation of these strings or rows of cells not only are the genital cells concerned, but also still undifferentiated mesoderm cells (Fig. 540, B, C), which originate from the dorsal wall of the cœlom-sacs and lie next to the genital cells. Some of these last tend to envelop the genital cells. We designate them the epithelial cells of the genital rudiments (ep), while others form a cellular cord which takes a position medial and ventral to the genital cells.

From the genital cells in the female arise only the egg-cells (and the nutritive cells in those forms which have such). The follicular epithelium of the egg-tube, on the other hand, also the corresponding cells of the terminal chamber, originate from the epithelial cells. Phyllodromia and Orthoptera in general, to which this description applies, show in this respect tolerably simple relations, since the germinal or terminal compartment of the ovary in them is composed of relatively few cells. In most other insects, and especially those which have a great number of food-cells in the ovary, the germinal chamber (Keimfach) is extraordinarily large.

The ventral cellular cord (cz) develops into the proximal part of the oviduct, which widens out and receives the single egg-tubes.

The cœlom-sacs in the farther course of their development, through the retrograde development of the parts extending into the appendages, through the development of the fat-bodies and through the delamination of the somatic and the splanchnic mesoderm layer, become greatly diminished in size. Finally, there remains left of them only a rather small cavity (c), which is bordered on the side by the rudiment of the pericardial septum (ps) and within by the terminal thread-plate (ef). The dorsally situated point where these two lamellæ pass into each other seems to stand in intimate connection with the cells of the rudiment of the heart (h). The cord-like genital rudiment hangs from the terminal thread-plate as from a mesentery (Fig. 549, gz).

Together with the growth of the primitive band around the yolk, and the formation of the back, the paired rudiments of the heart gradually extend to the neighborhood of the dorsal median line, followed by the genital rudiments which are connected with them by the terminal thread-plates. The genital rudiments advance thus to the dorsal side of the developing mid-intestine (Fig. 545, gz).

The terminal thread-plate (ef) is at first a simple epithelial plate. Soon, however, follows an arrangement of its cells whereby they appear to be arranged in vertical rows, each one of which corresponds to a developing ovarian tube. In this way the terminal thread-plate separates into the separate terminal threads of the ovarian tubes (Fig. 550, ef). In this process of division the uppermost dorsal edge of the terminal thread-plate takes no part. From it afterwards grows a thread which extends anteriorly, which becomes the common terminal thread of all the ovarian tubes, the so-called Müller’s thread. This is originally united with the pericardial septum, but seems in later stages to have no longer an intimate connection with it.

The formation of the single ovarian tubes, which in Phyllodromia number about 20, is accomplished by the extension of indentations from the dorsal side towards the ventral side of the ovarian rudiment (Fig. 550). At the same time the epithelial cells (ep), which were originally situated in part between the genital cells, become arranged in the form of an epithelium on the surface of the ovarian tubes, which soon forms on its outer surface a structureless cuticular tunica propria. The outer peritoneal membrane of the ovary becomes formed of the cells of the surrounding tissue of the fat-body.

The genital rudiment originally extends, as already stated, from the 2d to the 7th abdominal segment. In the last, however, the genital cells at first occur only sparingly, and afterwards completely disappear, so that here the genital cord appears composed of epithelial cells only. This part is the rudiment of the oviduct proper, and forms a direct continuation of the above-mentioned cell-cord which is situated ventralward from the genital cells, from which, as we have seen, the proximal cup-shaped section of the oviduct is formed. The hinder section of the oviduct turns down ventrally in order to unite at the boundary between the 7th and 8th abdominal segments with the hypodermis. The rudiment of the oviduct originally forms a solid strand of cells. Afterwards a cavity is formed by the separation of the cells.

In later stages there is a considerable shortening of the genital rudiment, so that it occupies a smaller number of abdominal segments than at first. At the same time the single ovarian tubes pass out of their originally vertical position into one more horizontal.

The paired connections of the rudiments of the oviducts with the hypodermis of the intersegmental furrow between the 7th and 8th abdominal segments reminds us of the conditions in the Ephemeridæ. This is the primitive condition in insects. In the female of Phyllodromia there is developed during larval life, from an ectodermal invagination, an unpaired terminal section of the genital passage, which becomes the genital pouch in which the egg-case (oötheca) is held. This genital pouch is formed, as Haase has already proved, by the withdrawal of the chitinous ventral plate of the 8th and 9th abdominal segment by invagination into the interior of the body.

The development of the efferent passages has been investigated by Nusbaum in the cockroach (Periplaneta) and in the Pediculina. He found that only the vasa deferentia and the oviducts arise from the hinder cord of the germs of the sexual glands, that is, out of the mesodermal rudiments, while the other parts of the sexual efferent apparatus (uterus, vagina, receptaculum seminis, ejaculatory duct, penis, and all the accessory glands) develop from the integumental epithelium and are of ectodermal origin. In fact, the unpaired parts (uterus, penis, receptaculum seminis, unpaired glands) have developed from paired rudiments, being outgrowths of the hypodermis. The hinder portions of the rudiments of the sexual glands approach these hypodermal growths and fuse with them. Through a median fusion of the paired hypodermal growths arise the germs of the unpaired organs. These observations are in complete agreement with the results at which Palmén arrived by anatomical investigation (see p. 492).

From the agreement of the position of the sexual openings in Phyllodromia with the conditions observed in the Ephemeridæ, with which the Perlidæ also agree, we conclude that in the entire group of insects an opening between the 7th and 8th abdominal segments is the primitive condition, and that only by a secondary shifting has a more posterior position of the opening (in many forms) been brought about. In this category we must certainly include the Thysanura, in which the sexual opening is single and situated between the 8th and 9th abdominal segments.

Development of the male germinal glands.—These rudiments arise in exactly the same manner as those of the female. Sexual differentiation takes place in the later embryonic stages. We then notice that in the male four masses of genital cells become surrounded by epithelial cells. These masses, which form the germs of the four testicular follicles of Phyllodromia, stand in intimate union with the rudiment of the vas deferens, and in the later stages move in connection with the latter, away from and behind the original genital rudiment. There remains, then, with the terminal thread-plate a remnant of the genital rudiment, which, according to Heymons, forms the female part of the original hermaphroditic genital rudiment, and in special cases may develop even into rudimentary egg-tubes and eggs. The rudimentary organ arising out of this genital rudiment may also be demonstrated in the adult male of Phyllodromia.

In the female the oviduct arises directly out of the originally established efferent passage. In the male, on the contrary, it is not, along its whole length, transformed into the vas deferens, but its distal terminal portion degenerates and is replaced by a newly formed terminal portion of the vas deferens, which then unites with the ectodermal ductus ejaculatorius. (Korschelt and Heider.)

On reviewing the facts as to the origin of the sexual organs, as in Phyllodromia,^[84] as just described, it will be seen that they afford proof that in the derivation of the genital cells from the epithelial cells of the cœlom-sacs, there is a direct agreement with the annelids. In the later development of the paired genital glands, and of an efferent passage standing in direct union with the glands themselves, there is a certain agreement with the conditions in Peripatus. In the first place, the dorsal position of the genital glands is the same in the two groups. On the other hand, the genital glands of Peripatus, according to Sedgwick, are formed by direct fusion of the successive cœlom-sacs (and a similar point of view has been taken by Heathcote for the myriopods), hence it results that in Peripatus the genital cavities arise out of the cœlom-cavities. In the insects, on the other hand, the genital rudiment lies, to be sure, in the wall of the cœlom-sac, but the genital cavity (lumen of the oviducts) in them arises separately from the cœlom-sacs, while the cœlom-cavities finally become a small part of the definite body-cavity. We must consider the conditions in Peripatus and the myriopods as the more primitive, directly pointing to the annelids; on the other hand, those of the insects as derived and secondary.

If we attempt to homologize the sexual efferent passages of insects with those of Peripatus, we are compelled to refer them to a modified pair of nephridia, and the origin of the latter (Peripatus) from the mesoderm agrees with that of insects. In general, however, in the development of the sexual outlets of insects, there are no characters which can be regarded as favorable to such a view. We must here accept the fact that the mode of development is secondary.

Mention should be specially made of the fact we owe to Heymons, that in the genital rudiment of Phyllodromia the genital cells and epithelial cells can be distinguished from each other from the very beginning. This fact does not favor the generally accepted view that the follicle-cells and egg-cells arise through a later differentiation from one and the same kind of cell. From their first origin, indeed, in Phyllodromia, both kinds of cells may be referred to the same source.

The mode of origin of the genital rudiments in Diptera and Aphides deserve special mention. In these groups the sexual germs are present in very early stages of life. This certainly in part is the result of the parthenogenetic and pædogenetic mode of reproduction in the two groups, which leads to an early differentiation of the sexual germs.

In the Diptera the first germs of the genital glands are represented by the polar cells (Fig. 551, pz). In the asexual developing eggs of the oviparous Cecidomyia larva, before the formation of the blastoderm, there separates from the hinder pole (D) a rather large cell rich in granules, which soon divides into two and afterwards four polar cells. After the completion of the blastoderm these polar cells then pass in among the blastoderm cells (G) and into the interior of the embryo, where they are in later stages symmetrically arranged in two groups, and, enveloped by the cells of surrounding tissues, transformed into the genital rudiments. (Metschnikoff.)

In Chironomus (Fig. 552, p), according to Balbiani, two polar cells almost simultaneously separate from the hinder pole of the egg, which, by division, form a group of four and eight cells. Exactly as in the case in Cecidomyia, these cells are taken within the embryo, where they lie divided into two groups on each side of the proctodæum. In all the young, freshly hatched larvæ; these two spindle-shaped groups, whose cells soon increase in number, may be seen situated dorsally on the side of the heart, enveloped by a clear cellular membrane which ends before and behind in a ligament-like terminal thread. The anterior terminal thread is the rudiment of the so-called Müller’s thread. The thread at the posterior end is the rudiment of the paired efferent passage of the genital glands. Through a division of the cells lying in the interior of the rudiments of the ovaries, there results the formation of a rosette-shaped group of cells which corresponds to the contents of an ovarian tube. With this view of Balbiani the later observations of Ritter agree.

As in the Diptera, so in the Aphides, the first germs of the genital organs are differentiated very early in life. In the early stage in which through an invagination from the hinder pole of the egg the first rudiment of the amnion-cavity is formed, a group of cells becomes separated from the wall of this invagination before the formation of the lower layer, which at this time lies as an unpaired roundish mass within the embryo. This group of cells, according to Balbiani and Witlaczil, has arisen by division of a single cell. Afterwards it becomes horseshoe-shaped and divides into a number of roundish masses of cells, which are arranged in similar numbers on each side of the median plane of the body, and form the rudiments of the terminal fan (Endfächer). They are covered by an epithelial envelope which passes anteriorly into the terminal threads, posteriorly into the efferent passage. The origin of this epithelial case is unknown. The efferent passages of the separate ovarian tubes are united into a common oviduct, and this fuses with an unpaired ectodermal invagination lying under the hind intestine from which the accessory sexual organs are formed. (Korschelt and Heider from Metschnikoff, Witlaczil, Will.)

In the Hymenoptera Ganin has observed in the embryo of Platygaster the rudiments of the sexual glands in the form of two rounded masses situated near the posterior intestine and apparently derived from the same blastems or buds as the latter.

Uljanin studied these organs in the larva of the honey-bee. They are two reniform bodies in the middle of which will soon appear the ovarian tubes. They also give birth to the internal parts of the excretory ducts, while the external part of the genital tube, as also the accessory glands which are connected with it, are derived by an invagination of the hypodermis at the surface of the penultimate segment.

Dohrn observed in the larva of ants the rudiments of the ovaries in the form of two pyriform masses, each with eight prolongations which he regarded as young ovarian tubes.

In Encyrtus Bugnion observed the rudiments of the sexual glands in the middle of the larval period; they were rounded and with no apparent connection with the neighboring organs. Afterwards these rudiments elongated, approached nearer to the ventral surface, and placed themselves in relation with some small cell-groups which appeared under the rectum, and seemed destined to form the efferent canal (vas deferens) and accessory glands of the genital organs. He thought the sex could be recognized in the second half of larval life, the male gland being distinguished by its rounded shape and smaller size; the ovary by its oval form and larger size. In larvæ ready to be transformed the testis formed a cellular mass enveloped by a cuticle, and at its hinder end prolonged into an epithelial cord, which is undoubtedly the vas deferens. The ovary had a similar envelope, and from its cellular mass arose epithelial cords which were destined to become the ovarian tubes.

m. Length of embryonic life

The duration of embryonic life varies greatly in different insects. The embryo of the blow-fly is fully developed in less than 24 hours, that of the house-fly in 24 hours. In the locusts and tree-cricket the embryos begin to develop at the end of the summer, continuing to grow until the cool weather of autumn, when growth is arrested, the later stages being finished in the latter part of the spring. It is so, likewise, with the embryos of many moths and other insects.

This has been observed only in a few cases, and careful observations as to the exact manner in which the embryo breaks the egg-shell and frees itself from the amnion are much needed. Also the rapid changes of form from that of the embryo within the egg-shell, and that which it immediately assumes after breaking forth from the shell and membranes, have yet to be observed; for these will undoubtedly be found to have special phylogenetic significance. Indeed, the phylogenetic importance of the latest embryonic changes in insects just entering on the nymph or the larval stages is very great, though little attention has as yet been bestowed upon the matter.

As regards the changes at the time of hatching, Wheeler tells us that the cockroach (Phyllodromia), shortly after leaving its narrow place in the egg-capsule, undergoes a peculiar change in shape. Before hatching, and when confined in the egg-shell, the body is about one-third as wide as thick; but soon after breaking out of the chorion its body is much flattened, its dorso-ventral diameter being only about a third as great as its greatest breadth. This shows that the flattened shape of the body of cockroaches, which adapts them for their life under bark and stones, is a very late inheritance, and that these insects have descended from those with more cylindrical bodies. The end of the body, also, which in the egg is bent underneath the abdomen, is, after hatching, bent dorsally, as indicated by the anal stylets, which now point directly upwards and outwards. The spines and claws are developed shortly before hatching. In the Locustidæ (Xiphidium, etc.) Wheeler has observed that the pleuropodia, or 1st pair of abdominal temporary embryonic appendages, are shed during hatching. All the other embryonic appendages have also disappeared, except those which persist and have rapidly become modified to form the cercopods, or the ovipositor.

In locusts, as we have observed^[85] in the case of Melanoplus spretus, the egg-shell bursts open at the head end, when the nymph, immediately after extricating itself from the egg, casts off a thin pellicle (the amnion), as we have also noticed in the case of the larvæ of the flea, currant saw-fly, and other insects. Before the amnion is cast off, the young nymph is almost motionless, but by slight movements of the body draws itself, in about five minutes, out of the amnion. The exact process of extraction is as follows: While it lies motionless, it puffs out the thin, loose skin connecting the back of the head with the front edge of the prothorax. The distention of this part probably ruptures the skin, which slips over the head, the body meanwhile curved over until the skin is drawn back from the head; when the latter is thrown back, it withdraws its antennæ and legs, and the skin is in a second of time pushed back to near the end of the abdomen; finally, it draws its hind tarsi out of the skin, and in a moment or two more the young locust frees itself, kicks away the cast skin, which resembles a little white crumpled pellet, and which has also been compared to a diminutive mushroom, and walks actively off,—sometimes, however, with the cast skin adhering to the end of the abdomen. Before the shedding of the amnion the body and legs are soft and flabby; immediately after, it walks firmly on its legs. All the eggs hatched—at least one or more hundreds—at about the same time, i.e. before 11 A.M.

The nymph of Stagmomantis carolina also sheds an amnion-skin, like that of the locust; but the embryo before casting it off is much elongated, and probably, like the European Mantis religiosa, the curious elongated embryos have the same singular habit of suspending themselves by threads, as shown in Fig. 554.

The account by Pagenstecher of the first ecdysis of the European Mantis was so extraordinary that we asked Professor Cockerell to collect the eggs of our Stagmomantis in New Mexico and send them to us. This he has kindly done, writing that he can “hardly recognize a true moult, since all that is cast off is the egg-membrane. In short, Pagenstecher’s account must be not a little fanciful, unless our insect differs very much in its development from Mantis religiosa. The main change is that after leaving the egg the thorax enormously elongates, producing a bulging out, and thrusting the head forward.” Our observations on the alcoholic specimens fully corroborate Cockerell’s conclusions. Pagenstecher’s figure of the embryo appears to be inaccurate. Sharp states that the hatching nymphs remain suspended for some days until the “first change of skin is effected.” This so-called “skin” is evidently the amnion.

The 17–year Cicada, after hatching, is enveloped by the amnion, from which it soon extricates itself, and then drops deliberately to the ground, “its specific gravity being so insignificant that it falls through the air as gently and as softly as does a feather.” (Riley.)

Other insects, as caterpillars, have room enough to turn around within their shell and to eat their way through the walls of the chorion.

The meat-fly, as we have observed, hatches in the following manner. The embryo moves to and fro, the body twisting until the exochorion is ruptured; the egg-shell splits longitudinally, and in one or two seconds the larva pushes its way out through the anterior end, and in a second or two more extricates itself from the shell. The latter scarcely changes its form, and the larva slips out, leaving the amnion within.

In the case of a fossorial wasp, Specius speciosus, which carries Cicadæ into its burrow, laying an elongated egg on the body under the median thigh of its victim, the larva on hatching, Riley states, “does not emerge from the skin of the egg, but merely protrudes its head and begins at once to draw nourishment from between the sternal sutures of the Cicada.”

The hatching spines.—Animals belonging to quite distinct classes are provided late in embryonic life with hard knobs or spines, which are temporary structures for the purpose of breaking or cutting open the egg-shell, when it is too thick and solid to be ruptured by the movements of the embryo. The embryos of certain lizards, turtles, the blind worm and some snakes, of the crocodile, and even birds, as well as the duckbill and Echidna, are provided with them, always occurring, so far as we are aware, on the end of the upper jaw. In the Arthropoda similar structures have thus far only been met with in myriopods and insects, though an analogous structure on the cephalothorax of the embryo of phalangids has been observed by Balbiani. Metschnikoff describes and figures a low conical spine serving this purpose situated on the embryonal cuticle over the head of the advanced embryo of Strongylosoma, and one on the 3d pair of mouth-parts of Geophilus.

In the winged insects, the embryo of Forficula is said by Heymons to bear a single spine between the eyes, which serves as an egg-tooth. The embryo of the Hemerobiidæ, according to Hagen, “opens the egg with an egg-burster like a saw.” (Proc. Bost. Soc. Nat. Hist., xv, p. 247.) Riley states that the egg-burster, or ruptor ovi, as he calls it, of Corydalus cornutus, has “the form of the common immature mushroom,” and he adds that it is a part of the amnion, being “easily perceived on the end of the vacated shell.” Wheeler has observed three pairs of broad-based chitinous “hatching spines” used by Doryphora in rupturing its embryonic envelopes, and which are secreted by pyramidal thickenings of the hypodermis (Figs. 555, 556).

The hatching spine of Pulex canis (Fig. 557) is a thin vertical plate, like the edge of a knife, situated in the median line of the head very near the posterior end, and is somewhat cultriform, the upper edge slightly hollow, and turned up a little at the anterior end. Though we did not see it working, it is situated at just the point on the head where it would come in contact with the egg-shell, and it was evident that the larva, by moving its head back and forth, would produce a slight split in the chorion and cause it to burst asunder. Later on in larval life it disappears, probably at the first moult.

LITERATURE ON EMBRYOLOGY

Koelliker, Albert. Observationes de prima insectorum genesi, etc. Turici, 1842, pp. 29, 3 Pls.

Rathke, H. Zur Entwickelungsgeschichte der Maulwurfsgrille (Gryllotalpa vulgaris). (Müller’s Archiv, 1844, ii, p. 27, Figs. 1–5.)

Zaddach, G. Untersuchungen über die Entwicklung und den Bau der Gliederthiere. I. Die Entwicklung des Phryganideneies. Berlin, 1854, pp. 138, 5 Taf.

—— Ueber die Entwicklung der Insekten. (Schrift, d. k. phys.-oekon. Gesell. Königsberg, viii Jahrg., 1867, Sitzb., p. 16.)

Leuckart, Rudolph. Die Fortpflanzung und Entwicklung der Pupiparen. Nach Beobachtungen an Melophagus ovinus. (Abhandl. Naturf. Gesell. Halle, iv, pp. 1–82, 3 Taf.) Halle, 1858.

Huxley, T. H. On the organic reproduction and morphology of Aphis. Pt. I, 1858; Pt. II, 1858. (Trans. Linn. Soc., xxii, pp. 193–219, 221–236, 5 Pls.)

Weismann, A. Die Entwicklung der Dipteren im Ei, nach Beobachtungen an Chironomus sp., Musca vomitoria und Pulex canis. (Zeitschr. f. wiss. Zool., xiii, 1863, pp. i-xvi, 1–263, 14 Taf.)

—— Zur Embryologie der Insecten. (Arch. f. Anat. u. Physiol., 1864.)

—— Beiträge zur Kenntnis der ersten Entwicklungsvorgänge im Insectenei. In: Beiträge zur Anatomie und Embryologie, etc. (Festschrift für J. Henle. Bonn, 1882.)

Kupffer, C. Ueber das Faltenblatt an den Embryonen der Gattung Chironomus. (Arch. Micr. Anat., ii, 1866.)

Metschnikoff, E. Embryologische Studien an Insecten. (Zeitschr. f. wiss. Zool., xvi, 1866, pp. 389–500, 10 Taf.)

Kupffer, Carl. De embryogenesi apud Chironomos Observationes, etc. Kiliæ, 1867, pp. 16, 1 Pl.

Packard, A. S. On the development of a dragon-fly (Diplax) [Æschna?]. (Proc. Bost. Soc. Nat. Hist., xi, 1868, pp. 366–372, 8 Figs.)

—— Embryology of Isotoma, a genus of Poduridæ. (Proc. Bost. Soc. Nat. Hist., xiv, 1870, pp. 13–15, 4 Figs.)

—— Embryological Studies on Diplax [Æschna?], Perithemis, and the thysanurous genus Isotoma. (Mem. Peabody Academy of Science, Salem, i, 1871, pp. 1–21, 3 Pls.)

—— Embryological studies on hexapodous insects. (Ibid., 1872, pp. 1–17, 3 Pls.)

—— The embryological development of the locust. (Chap. X, Third Report U. S. Ent. Commission, Washington, 1883, pp. 263–282, 7 Pls.)

Brandt, A. Beiträge zur Entwicklungsgeschichte der Libelluliden und Hemipteren. (Mém. Acad. St. Pétersbourg (7), xiii, 1869, pp. 1–33, 3 Taf.)

—— Ueber das Ei und seine Bildungsstätte. (Leipzig, 1878, pp. 196, 4 Taf.)

—— Commentare zur Keimbläschentheorie des Eies. I. Die Blastodermelemente und Dotterballen der Insecten. (Arch. f. Micr. Anat., 1880, xvii.)

Melnikow, N. Beiträge zur Embryonalentwicklung der Insecten. (Arch. f. Naturg., xxxv, 1869, pp. 137–189, 4 Taf.)

Bütschli, O. Zur Entwicklungsgeschichte der Biene. (Zeitschr. f. wiss. Zool., xx, 1870, pp. 519–564, 4 Taf.)

—— Bemerkungen über die Entwicklungsgeschichte von Musca. (Morph. Jahrb., xiv, 1888, pp. 170–174, 3 Figs.)

Kowalevsky, A. Embryologische Studien an Würmern und Arthropoden. (Mém. Acad. St. Pétersbourg (7), xvi, 1871, pp. 1–70, 12 Taf.)

—— Zur embryonalen Entwicklung der Musciden. (Biol. Centralbl., vi, 1886, pp. 49–54.)

Müller, Fritz. Beiträge zur Kenntniss der Termiten. (Jena. Zeitschr. Wissens., ix, 1875, pp. 241–263, 4 Taf.)

Oulganine, W. N. (also spelled Uljanin). Sur le développement des Podurelles. (Arch. Zool. Expér., iv, 1875, pp. xxxix-xl, und v, 1876, pp. xvii-xix.)

—— Beobachtungen über die Entwicklung der Poduren. (Russian.) (Nachr. k. Gesellsch. Freunde Naturw., Anthrop. und Ethnogr., xvi, 1875.)

Dohrn, A. Notizen zur Kenntniss der Insectenentwicklung. (Zeitschr. f. wiss. Zool., xxvi, 1876, pp. 112–138.)

Hatschek, B. Beiträge zur Entwicklungsgeschichte der Lepidopteren. (Jena. Zeitschr. f. Naturw., xi, 1877, pp. 38, 3 Taf., 2 Figs.)

Bobretzky, N. Ueber die Bildung des Blastoderms und der Keimblätter bei Insecten. (Zeitschr. f. wiss. Zool., xxxi, 1878, pp. 195–215, 1 Taf.)

Graber, Vitus. Vorläufige Ergebnisse einer grösseren Arbeit über vergl. Embryologie der Insecten. (Arch. f. Micr. Anat., xv, 1878, pp. 630–640, 1 Fig.)

—— Ueber die Polypodie bei Insectenembryonen. (Morph. Jahrb., xiii, 1888, pp. 586–615, 2 Taf.)

—— Ueber die primäre Segmentirung des Keimstreifs der Insecten. (Morph. Jahrb., xiv, 1888, pp. 345–368, 2 Taf., 4 Figs.)

—— Vergleichende Studien über die Keimhüllen und die Rückenbildung der Insecten. (Denkschr. Acad. Wiss. Wien., lv, 1888.)

—— Vergleichende Studien über die Embryologie der Insecten und insbes. der Musciden. (Denkschr. Acad. Wiss. Wien., lvi, 1889.)

—— Ueber den Bau und die phylogenetische Bedeutung der embryonalen Bauchanhänge der Insekten. (Biol. Centralbl., ix, 1889, pp. 355–363.)

—— Vergleichende Studien am Keimstreif der Insecten. (Denkschr. Acad. Wiss. Wien., lvii, 1890.)

—— Üeber die embryonale Anlage des Blut- und Fettgewebes der Insecten. (Biol. Centralbl., xi, 1891, pp. 212–224.)

—— Zur Embryologie der Insekten. (Zool. Anzeiger, xiv, 1891, pp. 286–291.)

—— Über die morphologische Bedeutung der ventralen Abdominalanhänge der Insekten-Embryonen. (Morph. Jahrb., xvii, 1892, pp. 467–482.)

Barrois, J. Développement des Podurelles. (Assoc. Franc. p. l’Avance. des Sc., 7^e Sess., 1879.)

Kadyi, H. Beitrag zur Kenntnis der Vorgänge beim Eierlegen der Blatta orientalis. (Zool. Anzeiger, 1879, ii, pp. 632–636.)

Tichomiroff, A. Ueber die Entwicklungsgeschichte des Seidenwurms. (Zool. Anzeiger, ii Jahrg., 1879, pp. 64–67.)

—— Zur Entwicklungsgeschichte des Seidenspinners (Bombyx mori) im Ei. (Arb. Laborat. Zool. Mus. Moskau, i, 1882, pp. vii, v, 1–80, 3 Taf., 48 Figs.) (Russian.)

—— Ueber die Entwicklung der Calandra granaria. (Biol. Centralbl., x, 1890, p. 424.)

Balfour, Francis M. A treatise on comparative embryology. i, ii. London, 1880. 2d edit., London, 1885.

Hertwig, O. und R. Die Coelomtheorie. Versuch einer Erklärung des mittleren Keimblattes. Jena, 1881. (Jena. Zeitschr., xv, pp. 150, 3 Taf.)

Selvatico, D. S. Sullo sviluppo embrionale dei Bombicini. (Boll. Bachicoltura, viii, 1881.)

Lemoine, V. Recherches sur le développement des Podurelles. (Assoc. Franç. pour l’Avanc. d. Sc. Congrès de la Rochelle, 1882.)

Balbiani, E. G. Sur la signification des cellules polaires des Insectes. (Compt. rend. Ac. Sc. Paris, xcv, 1882.)

—— Contribution à l’étude de la formation des organes sexuels chez les Insectes. (Recueil Zool. Suisse, ii, 1885.)

Korotneff, A. Entwicklung des Herzens bei Gryllotalpa. (Zool. Anzeiger, vi Jahrg., 1883, pp. 687–690, 2 Figs.)

—— Die Embryologie der Gryllotalpa. (Zeitschr. f. wiss. Zool., xli, 1885, pp. 570–604, 3 Taf.)

Nusbaum, J. Vorl. Mittheilung über die Chorda der Arthropoden. (Zool. Anzeiger, vi Jahrg., 1883, pp. 291–295, 3 Figs.)

—— Die Entwicklung der Keimblätter bei Meloë proscarabæus. (Biol. Centralbl., viii, 1888, pp. 449–452, 2 Figs.)

—— Zur Frage der Segmentirung des Keimstreifs und der Bauchanhänge der Insectenembryonen. (Biol. Centralbl., ix, 1889, pp. 516–522, 1 Fig.)

—— Zur Frage der Rückenbildung bei den Insectenembryonen. (Biol. Centralbl., x, 1890, pp. 110–114.)

—— Ueber die Entwicklungsgeschichte der Ausführungsgänge der Sexualdrüsen bei den Insecten. (In Polish, with German résumé, pp. 39–42.) (“Kosmos” Lemberg, 1884, ix Jahrg.)

—— Zur Embryologie des Meloë proscarabæus, Marscham. (In Polish, with Latin explanation of plates.) (“Kosmos” Lemberg, 1891.)

Schneider, A. Ueber die Entwicklung der Geschlechtsorgane der Insecten. (Zool. Beiträge, herausg. v. A. Schneider, i, 1883.)

Will, L. Zur Bildung des Eies und des Blastoderms bei den viviparen Aphiden. (Arb. Zool. Zoot. Inst. Würzburg, vi, 1883, pp. 217–258, 1 Taf.)

—— Entwicklungsgeschichte der viviparen Aphiden. (Spengel’s Zool. Jahrbücher. Abth. f. Anat. und Ont., iii, 1888, pp. 201–286, 5 Taf.)

Patten, W. The development of Phryganids, with a preliminary note on the development of Blatta germanica. (Quart. Journ. Micr. Sc., xxiv, 1884, pp. 54, 3 Pls.)

—— Studies on the eyes of Arthropods. I. Development of the eyes of Vespa, with observations on the ocelli of some insects. (Journ. of Morphol., Boston, i, pp. 193–226, 1 Pl.)

—— Studies on the eyes of Arthropods. II. Eyes of Acilius. (Journ. of Morphol., Boston, ii, 1888, pp. 97–190, 7 Pl., 4 Figs.)

—— Eyes of molluscs and Arthropods. (Mitth. Zool. Station Neapel., vi, 1888, pp. 542–756, 5 Pls.)

Grassi, B. Interno allo sviluppo delle api nell’ uovo. (Atti Acad. Gioenia. Scienc. Nat. Catania (3), xviii, 1884.)

—— Breve nota intorno allo sviluppo degli Japyx. Catania, 1884, also in I progenitori degli Insetti e dei Miriopodi. 1. L’ Japyx e la Campodea. (Atti Acad. Gioenia Sc. Nat. Catania (3), xix, 1885.)

Ayers, H. On the development of Œcanthus niveus and its parasite Teleas. (Mem. Boston Soc. Nat. Hist., iii, 1884, pp. 225–281, 8 Pls.)

Witlaczil, Em. Entwicklungsgeschichte der Aphiden. (Zeitschr. f. wiss. Zool., xl, 1884, pp. 559–696, 9 Taf.)

Hallez, P. Orientation de l’embryon et formation du cocon chez la Periplaneta orientalis. (Compt. rend. Ac. Sc. Paris, ci, 1885, pp. 444–446.)

—— Sur la loi de l’orientation de l’embryon chez les Insectes. (Compt. rend. Ac. Sc. Paris, ciii, 1886, pp. 606–608.)

Heider, K. Ueber die Anlage der Keimblätter von Hydrophilus piceus. (Abh. k. Acad. Wiss. Berlin, 1885.)

—— Die Embryonalentwicklung von Hydrophilus piceus L. (I. Theil. Jena, 1889, pp. 1–98, 13 Taf., 9 Figs.)

Carrière, J. Kurze Mittheilungen aus fortgesetzten Untersuchungen über die Sehorgane. 7, Die Entwicklung und die verschiedenen Arten der Ocellen. (Zool. Anzeiger, ix Jahrg, 1886, pp. 141–147, 479–481, 496–500.)

—— Die Entwicklung der Mauerbiene (Chalicodoma muraria Fabr.) im Ei. (Arch. f. Micr. Anat., xxxv, 1890, pp. 141–165, 1 Taf.)

—— Die Drüsen am ersten Hinterleibsringe der Insectenembryonen. (Biol. Centralbl., xi, 1891, pp. 110–127, 3 Figs.)

Miall, L. C., and Denny, A. The structure and life-history of the cockroach (Periplaneta orientalis). London, 1886. (The section on embryology by J. Nusbaum.)

Ryder, J. The development of Anurida maritima Guerin. (Amer. Naturalist, xx, 1886, pp. 299–302, 1 Pl)

Stuhlmann, F. Die Reifung des Arthropodeneies nach Beobachtungen an Insecten, Spinnen, Myriopoden und Peripatus. (Ber. Freib. Naturf.-Gesellsch., i, 1886.)

Blochmann, F. Ueber die Richtungskörper bei Insecteneiern. (Morph. Jahrbuch, xii, 1887, pp. 544–574, 2 Taf.)

Bruce, Adam Todd. Observations on the embryology of insects and arachnids. A memorial volume. Baltimore, 1887, 4º, pp. 31, 6 Pls.

Weismann, A., und Ischikawa, Ch. Ueber die Bildung der Richtungskörper bei thierischen Eiern. (Ber. Naturf. Ges. Freiburg, iii, 1887.)

Cholodkowsky, N. Ueber die Bildung des Entoderms bei Blatta germanica. (Zool. Anzeiger, xi Jahrg., 1888, pp. 163–166, 2 Figs.)

—— Studien zur Entwicklungsgeschichte der Insecten. (Zeitschr. f. wiss. Zool., xlviii, 1889, pp. 89–100, 1 Taf.)

—— Zur Embryologie von Blatta germanica. (Zool. Anzeiger, xiii Jahrg., 1890, pp. 137–138.)

—— Zur Embryologie der Hausschabe (Blatta germanica). (Biol. Centralbl., x, 1890, p. 425.)

—— Ueber die Entwicklung des centralen Nervensystems bei Blatta germanica. (Zool. Anzeiger, xiv Jahrg., 1891, pp. 115–116.)

—— Zur Embryologie der Insecten. (Ibid., 1891, pp. 465–466.)

—— Die Embryonalentwicklung von Phyllodromia (Blatta germanica). (Mém. Acad. St. Pétersbourg (7), xxxviii, 1891.)

Henking, H. Die ersten Entwicklungsvorgänge im Fliegenei und freie Kernbildung. (Zeitschr. f. wiss. Zool., xlvi, 1888, pp. 289–336, 4 Taf., 3 Figs.)

—— Ueber die Bildung von Richtungskörpern in den Eiern der Insekten und deren Schicksal. (Nachr. Ges. Wiss. Göttingen, 1888.)

Platner, G. Die erste Entwicklung befruchteter und parthenogenetischer Eier von Liparis dispar. (Biol. Centralbl., viii, 1888, pp. 521–524.)

Schmidt, F. Die Bildung des Blastoderms und des Keimstreifs der Musciden. (Sitz. Naturf. Ges., Dorpat, viii, 1889.)

Viallanes, H. Sur quelques points de l’histoire du développement embryonnaire de la Mante religieuse. (Rec. Biol. du Nord de la France, ii, 1889–1890.)

Voeltzkow, A. Entwicklung im Ei von Musca vomitoria. (Arb. Zool. Zoot. Inst. Würzburg, ix, 1889, pp. 1–48, 4 Taf.)

—— Melolontha vulgaris, ein Beitrag zur Entwicklung im Ei bei Insecten. (Arb. Zool. Zoot. Inst. Würzburg, ix, 1889, pp. 49–64, 1 Taf.)

Wheeler, W. M. Ueber drüsenartige Gebilde im ersten Abdominalsegment der Hemipterenembryonen. (Zool. Anzieger, xii, 1889, pp. 500–504, 2 Figs.)

—— The embryology of Blatta germanica and Doryphora decemlineata. (Journ. of Morph., Boston, iii, 1889, pp. 291–374, 7 Pls., 16 Figs.)

—— On the appendages of the first abdominal segment of the embryo cockroach (Blatta germanica). (Proceed. Wis. Acad. Science, Arts, and Letters, viii, 1890, pp. 87–140, 3 Pls.)

—— Ueber ein eigenthümliches Organ im Locustidenembryo (Xiphidium ensiferum). (Zool. Anzieger, xiii, 1890, pp. 475–480.)

—— Neuroblasts in the Arthropod embryo. (Journ. of Morphol., iv, 1891, pp. 337–343, 1 Fig.)

—— A contribution to insect embryology. In. Diss. (Journ. Morph., Boston, viii, 1893, pp. 1–160, 6 Pls., 7 Figs.)

Heymons, R. Ueber die hermaphroditische Anlage der Sexualdrüsen beim Männchen von Phyllodromia (Blatta) germanica. (Zool. Anzeiger, xiii Jahrg., 1890, pp. 451–457, 3 Figs.)

—— Die Entstehung der Geschlechtsdrüsen von Phyllodromia (Blatta) germanica L. In. Diss. Berlin, 1891.

—— Die Embryonalentwickelung von Dermapteren und Orthopteren unter besonderer Berücksichtigung der Keimblätterbildung. Monographisch bearbeiteit, 12 Lith. Taf. und 33 Figs. Jena, 1895, 4º, pp. 136.

—— Ueber die Fortpflanzung und Entwickelungsgeschichte der Ephemera vulgata L. (Sitzungsb. Gesell. Naturf. Freunde, Berlin, 1896, pp. 81–96.)

—— Entwicklungsgeschichtliche Untersuchungen an Lepisma saccharina L. (Zeitschr. wiss. Zool., lxii, 1897, pp. 588–631, 2 Taf.)

Ritter, R. Die Entwicklung der Geschlechtsorgane und des Darmes bei Chironomus. (Zeitschr. f. wiss. Zool., 1, 1890, pp. 408–427, 1 Taf.)

Tichomirowa, O. S. Zur Embryologie von Chrysopa. (Biol. Centralbl., x, 1890, p. 423.)

Pedaschenko, D. Sur la formation de la bandelette germinative chez Notonecta glauca. (In Russian.) (Revue Sc. Natural. St. Pétersbourg, i, 1891.)

Korschelt, E., and Heider, K. Lehrbuch der vergleichenden Entwicklungsgeschichte der wirbellosen Thiere. (Spec. Th. Heft ii, Jena, 1891, many Figs.)

With the writings of Lang (Comp. Anatomy).