Fig. 482.—Female Dyticus, laying eggs: A, ovipositor extended. B, egg of Notonecta, attached to stem of rush. C, egg of Dyticus, laid in excavation in rush.—After Régimbart, from Miall.
Insects as a rule arise from eggs which are laid in a great variety of situations, those species which are viviparous being exceedingly few in number compared with the class as a whole. It is noteworthy that Leydig has found in the same Aphis, and even in the same ovary, an egg-tube producing eggs, while a neighboring tube was producing viviparous individuals.[77] The viviparous species are confined to certain May-flies, the Aphidæ, Diptera (Sarcophaga, Tachinidæ, Œstridæ, and Pupipara), and to certain Coleoptera (Stylopidæ and some Staphylinidæ).
The number of eggs laid varies from a very few, as in the Collembola and in the Psocidæ, or 15 or even less in certain fossorial wasps, and from 20 to 35 in some locusts to many thousands in the social insects, the honey-bee laying by estimate over 1,000,000 eggs in the course of her life. Dr. Sharp thinks that from 50 to 100 may perhaps be taken as an average number for one female to produce. The eggs of insects with a complete metamorphosis are said by Brauer to be smaller in proportion to the parent than those laid by ametabolous or heterometabolous insects. In this respect the insects are paralleled by the birds, the highest forms laying smaller eggs than the water birds, ostrich, Apteryx, etc.
Fig. 483.—Eggs (e) of Hydrobius (?) and their capsules, from which the larva, Fig. 452, hatched.—Emerton del.
The egg, or ovum, when laid is not always ripe or perfect, but, as in those of ants, continues to grow after oviposition. Others are laid some time after the embryo has begun to form; and in the flesh-flies the larva hatches before the egg is deposited.
Fig. 484.—Egg-masses of Chironomus: A, string of eggs of C. dorsalis, divided into sections to show both sides. B, twisted fibres which traverse the string of eggs. C, egg-mass of Chironomus (sp). D, egg-mass of a third species. E, part of D, more highly magnified. F, developing eggs, two stages.—After Miall.
Insects as a rule instinctively lay their eggs near or upon objects destined to be the food of the larva; those of caterpillars on leaves, those of many flies on meat or carrion, those of Copris and other dung-beetles in dung, those of aquatic insects in water, while many oviposit in the earth or in plants (Fig. 482), or in the bodies of animals destined to be the hosts of the parasitic larvæ. As the eggs are preyed upon by mites and other animals, the contrivances and modifications of the mode of egg-laying, and the situations in which they are placed, are almost endless. Many insects lay their eggs in a mass, covered with a gummy substance; or those laid in the water, as the eggs of dragon-flies, caddis-flies, Chironomus (Fig. 484), etc., are enveloped by a jelly-like mass.
Fig. 485.—Egg-capsule of Periplaneta americana: a, side; b, end view; c, natural size.—After Howard and Marlatt, Bull. 4, Div. Ent. U. S. Dept. Agr.
The oötheca of the cockroach (Fig. 485) is a solid, dense case, which, after being carried about by the mother, can be left without harm in the crevices of the floors of houses. The oötheca of Mantis (Fig. 486) is formed by a large mass of frothy matter, which hardens and is attached to stems of plants.
Fig. 486.—Egg-capsules of Mantis carolina.—After Riley.
On the other hand, the female “walking-stick” (Diapheromera femoratum) drops her eggs, says Riley, loosely upon the ground, from whatever height she may happen to be, and “one hears a constant pattering, not unlike drops of rain, that results from the abundant dropping of these eggs, which, in places, lay so thick among and under the dead leaves that they may be scraped up in great quantities.” (Report for 1879.)
The eggs of the lace-winged flies are supported on pedicels, above the reach of ovivorous mites.
The female Chrysopa usually lays between 40 and 50 eggs. In one case, we observed that 18 egg-stalks were deposited, but there were only nine well-formed eggs in the batch, and nine eggless stalks, some only half the usual height, others with the knob of cement at the end to which the egg is ordinarily fastened. The eggs are evidently stuck on to the end of the pedicel after the latter has been formed, as, in one instance, an egg was glued to the stalk very much out of centre, the insect’s abdomen not having been aimed straight, so to speak, at the mass of cement.
Fig. 487.—Eggs of Chrysopa, with larva and fly.
The eggs of Rhodites are fixed to a long stalk thickened at the end; those of Inquilines and certain Chalcids (Leucospis gigas, Fig. 489, A) are also stalked; and the use of this stalk in the eggs of Cynips (E) is thought by Adler to be respiratory, while, also, he states that the egg-cavity communicates with the egg-stalk, so that a part of the egg-contents can pass into the latter, and this happens at the laying of each egg. The egg of certain ichneumons (Paniscus, Fig. 488) ends in a short stalk, which is inserted in the skin of the caterpillar destined to serve as the host of the parasite, the eggs, as stated by De Geer, being retained more firmly in the integument by the stalk so swelling as to form two knobs (Fig. 498, c).
Fig. 488.—Young larva of Paniscus in position of feeding on the skin of a caterpillar: a, the egg-shell.—After Newport, from Sharp.
Certain Homoptera also have stalked eggs, as those of Psylla pyricola (Fig. 489, B), those of Aleyrodes citri (C, a, b), and of an allied form, Aleurodicus cocois (D), and those of Corixa (Fig. 493).
Fig. 489.—Stalked eggs: A, of a Chalcid (after Fabre); B, of Psylla (after Slingerland); C, of Aleyrodes; D, of Aleurodicus (after Riley and Howard); E, of Dryophanta scutellaris (after Adler).
Fig. 490.—Eggs of ox bot-fly, enlarged.—After Riley.
Reference should also be made to the eggs of lice, which are oval and attached to the hairs of their host. Those of the ox bot-fly (Hypoderma lineata) are usually placed four to six together, and fastened to a hair. The lower portion of the egg is admirably adapted for clasping a hair. “It consists of two lobes, forming a bulbous enlargement, which is attached to the egg by a broad, but rather thin, neck, so that, when the latter is viewed sidewise, it appears as a slender pedicel” (Fig. 490, a-d). (Riley in Insect Life, iv, p. 307.) The egg of another fly (Drosophila ampelophila, Fig. 491) bears a pair of long, slender appendages near the anterior end. “The egg is inserted into the soft pulp of the decaying fruit; these appendages leave the ovipositor last, and are spread out upon the surface of the mass. They, in this way, serve to keep the egg in place, and thus insure the emergence of the larva into the open air instead of into the more or less fluid mass in which the egg is situated. The larva issues from the egg just above the base of these appendages.” (Comstock.)
Fig. 491.—Egg of Drosophila.—After Comstock.
Mode of deposition.—The exact process of oviposition has been rarely observed, or at least not observed in detail, and further observations are much needed. In the cockroach (Phyllodromia), Wheeler has seen the eggs pass out of the oviduct and become arranged in the oötheca, in a way similar to that in the account published by Kadyi on Periplaneta.
Fig. 492.—Rocky Mountain locust (aa) depositing its eggs (c); d, the earth partially removed, showing (e) an egg-mass already in place, and (d) one being placed; f shows where such a mass has been covered over. A, oviposition; j, position of oviduct; g, egg-guide; e, egg. B, egg-mass of the same; a, from side, b, from beneath, c, from above.—After Riley.
“When about to form the capsule, the female Blatta closes the genital armature, and the two folds of the white membrane which lines the oöthecal cavity close vertically in the middle line. Then some of the contents of the colleterial glands are poured into the chamber, and bathe the inner surface of the posterior wall. The first egg glides down the vagina from the left ovary, describes an arc, still keeping its germarium-pole uppermost, after having pressed the micropylar area against the mouth of the spermatheca, passes to the right side of the back of the chamber, and is placed perpendicularly two-thirds to the right of the longitudinal axis of the insect’s body. The next egg comes from the right ovary, describes an arc to the opposite side of the body, decussating with the path of the first egg, and is placed completely on the left side of the median line. The third egg comes from the left ovary, and is made to lie completely on the right side of the median line; and so the process continues, the ovaries discharging the eggs alternately, and each egg describing an arc to the opposite side of the capsule. The oöthecal chamber soon becomes too small to contain all the constantly accumulating eggs, so the anal armature opens and allows the end of the capsule to project. A raised line, the impression of the edges of the white membrane, runs down the end of the capsule. The last egg deposited comes from the right ovary, and lies two-thirds on the left, and one-third to the right, of the median line. As soon as the egg is laid, a further discharge from the colleterial glands spreads over the vaginal or anterior wall of the cavity, and becomes evenly continuous with the secretion which has before been spread over the back and the sides of the capsule by the white membrane.
“The crista, a cord-like ridge running the full length of the dorsal surface of the capsule, is a thick-walled tube, either half of which is formed by the edge of the side walls of the capsule split into two laminæ. The rhythmical clasping of the three pairs of palpi which guard the vaginal opening is registered in an exquisite pattern on the inner face of either half of the crista.”[78]
The mode of oviposition in the locust has been fully described by Riley, who states that the eggs pass down and out of the oviduct, and “guided by a little finger-like style” (Fig. 298), they pass in between the horny valves of the ovipositor, and issue at their tips amid the mucous fluid which forms the egg-capsule (Fig. 492).
Vitality of eggs.—It is well known that the eggs of phyllopod and other fresh-water Crustacea have wonderful vitality, withstanding extreme dryness for several years, at least from two to ten. Such cases are unknown among insects. It has been observed, however, by T. W. Brigham, and also by L. Trouvelot, that the eggs of the walking-stick (Diapheromera femorata) for the most part hatch only after the interval of two years.[79]
The eggs of Bittacus are said by Brauer to lie over unhatched for two years; indeed, the first condition of their hatching is a complete drying of the earth in which the eggs lie, the second is a succeeding thorough wetting of the ground in spring.
Appearance and structure of the ripe egg.—The eggs of insects are on the whole rather large in proportion to the size of the parent, especially so in many minute forms, as the fleas, lice, etc.
Their general shape is spherical or oval, often cylindrical; where the eggs are long and cylindrical a dorsal and ventral side can be distinguished (Fig. 502). They are in the Tortricidæ and Limacodid moths flattened, thin, and scale-like. In the eggs of locusts and grasshoppers, as well as certain Diptera, the ventral side of the embryo corresponds to the convex side, and the concave side of the egg to the dorsal region of the embryo (Figs. 502 and 493).
There is an anterior and posterior end or pole, the anterior end being that which in the body of the parent lies towards her head, or towards the upper or distal end of the ovarian tube. Towards this end lies in the later stages of embryonic life the head-end of the embryo, while the posterior end of the embryo is turned towards the hinder pole of the egg (Figs. 493 and 520).
The egg-shell and yolk-membrane.—The ripe egg is protected by two membranes: 1, an inner or vitelline membrane or oölemma (dh) (Fig. 500, d), produced in the egg by a hardening of the outer layer, and 2, the outer or chorion (c), which is secreted by the cells of the ovarian follicle. The latter is divided into two layers: an inner, the endochorion, and an outer, the exochorion.
Fig. 503.—Fertilization of the egg of a round-worm (Ascaris megalocephala): A, the ends (centrosomes) of the spindle formed. B, the spindle completed; sp, sperm-nucleus, with its chromosomes; ei, egg-nucleus; p, polar bodies.—After Boveri, from Field’s Hertwig.
Fig. 493.—Eggs of Corixa: A, early stage before formation of the embryo, from one side. B, the same viewed in the plane of symmetry. C, the embryo in its final position; a, anterior, p, posterior, end; l, left, r, right, v, ventral, d, dorsal, aspect. (The letters refer to the final position of the embryo, which is nearly diametrically opposite to that in which it first develops); m, micropyle; p, pedicle.—After Metschnikoff, from Wilson.
Fig. 494.—Eggs of Phasmidæ: A, Lonchodes duivenbodi. B, Platycrania edulis. C, Haplopus grayi. D, Phyllium siccifolium.—After Kaup, from Sharp.
While the yolk-membrane is usually a completely homogeneous, thin, structureless membrane, the chorion or shell of the egg is usually covered with a network of ridges enclosing polygonal areas, varying in shape according to the species or genus. These external markings are due to the impress of the cellular structure of the epithelium of the ovarian follicle.
In the chorion of the cockroach the surface appears to be finely granular, the finest granules being arranged in large, more or less regularly hexagonal areas, which are bounded by narrow, dark spaces, containing somewhat larger though less dense granules. The surface of the eggs of certain Phasmids are variously sculptured (Fig. 494).
The true structure of the chorion can only be, as Wheeler observes, seen in cross-sections, as shown by Blochmann, and also by Wheeler. The chorion consists of two chitinous laminæ kept in close apposition by means of numerous minute trabeculæ or pillars. It is the ends of these pillars that look like granules. In the spaces between the hexagonal areas, the trabeculæ are more scattered and individually thicker than those of the hexagons.
Fig. 495.—Egg of cotton-worm moth, Aletia: a, top view, showing the micropyle.—After Comstock.
Fig. 496.—Egg of Danais archippus.—After Riley.
These markings are of singular beauty and complexity in the eggs of many Lepidoptera, whose ova are variously ribbed, forming a beautiful fretwork of raised lines (Figs. 495 and 496), while in the Diptera and Hymenoptera the chorion is less solid, and usually smooth under low powers. The exochorion of the egg of the house and meat fly (C. vomitoria) is pitted with elongated hexagonal depressions, which cross the egg transversely. That of the honey-bee is also divided into long hexagonal areas (Fig. 497).
Fig. 497.—Egg with embryo of honey-bee, × 40: ch, chorion; ga, ganglia; s. ga, brain; jm, jaw-muscles forming; c, œsophageal collar; fb, fore intestine; mb, mid-intestine; ab, hind-intestine.—After Cheshire.
Fig. 498.—Micropyle (Mk) of eggs; a, of a fly, Antomyia; b, Drosophila cellaris; c, stalked egg of Paniscus testaceus.—After Leuckart, from Perrier.
When the eggs are deposited in exposed places, and remain in such situations for several days, or weeks, or even through the winter, the shell is either solid and strengthened by the ribs and ridges; or the shell, if of winter eggs, is unornamented, and is dense and solid, to resist extremes in temperature or the attacks of egg-eating birds, mites, etc.
The micropyle.—This is an opening or canal, or, as in most insects, a group of canals situated at the anterior end of the egg for the entrance of the spermatozoa during the process of fertilization of the ovum (Fig. 498). In Acrydians, however, the micropyle is situated at the posterior end of the egg. The micropyle (Fig. 499) is a complicated apparatus within whose circumference the vitelline membrane appears to be firmly attached to the chorion, so that the perforation passes through the chorion as well as the yolk-membrane.
The micropyles of the cockroach are probably as simple and generalized as in any insect. Wheeler states that they are in Phyllodromia scattered over the end of the egg, “over a quadrant of the upper hemisphere, where the beautiful hexagonal pattern of the chorion gives away to an even trabeculation.” The micropyles are wide-mouthed, very oblique, funnel-shaped canals, perforating the chorion, the apertures of the funnels appearing under a low power as clear, oval spots, the long axis of which is parallel to the long axis of the egg.
Fig. 499.—a, fragment of a micropylar papilla, showing its lumen; b, optical section of another papilla, in this one the lumen extends to the vitelline membrane, but does not pass beyond it; c, d, e, and f, papillæ of different forms. A, anterior end of an ovarian egg, showing mode of growth of the micropylar papillæ: a, b, two successive stages; c, surface view of modified papillæ from the lower edges of the cap; d, tunica propria of the ovariole; e, remnant of the cell-mass that secreted (?) the micropylar cap.—After Ayers.
Fig. 500.—Egg of Perla maxima: c, chorion; d, oölemma; gs, glass-like covering of micropyle; l, cavity under same; g, canals penetrating chorion.—After Imhof, from Sharp.
“With a higher power the tube of each funnel is clearly visible as a thin canal which dilates rapidly into the large oval aperture on the outer face of the chorion. The narrow tube is sometimes fully as long as the large orifice. The micropylar perforations are all directed from the germarium to the vaginal pole of the egg. Hence a line, the hypothetical path of the spermatozoön, drawn through one of these oblique micropyles, and continued into the egg, would strike the equatorial plane. The female pronucleus, as we shall see further on, moves in this plane.” (Wheeler, p. 289.)
Fig. 501.—Micropyles: a, of Nepa cinerea; b, of Locusta viridissima; c, of a bug (Pyrrhocoris apterus).—From Gerstäcker.
The micropylar region is generally, at least in Orthoptera and Odonata, covered by a gelatinous cap (Figs. 499 and 500, gs), which may form a covering membrane which extends over a large part of the egg, or may envelop the entire outer surface. In some cases micropyles are scattered over the entire surface of the egg, but usually the perforation is situated at the end, and is often guarded by raised processes, either one or several, like bristles, or toadstools, etc., these being especially characteristic of the eggs of certain Hemiptera (Nepa, Fig. 501, a, and Ranatra), or the region is variously sculptured, as in the eggs of butterflies. In the micropylar apparatus of Œcanthus the papillæ have a distinct lumen (Fig. 499), or a channel for the ingress of the male filament.
Fig. 502.—Diagrammatic median section through egg of Musca in stage of fertilization (incorporating the figures of Henking and Blochmann): ch, chorion; d, dorsal; v, ventral side of the egg; dh, yolk-membrane; do, nutritive yolk; g, gelatinous cap over the micropyle (m); K, outer layer of plasma (Keimhautblastem); p, male and female pronucleus before copulation; r, directive body (Richtungskörper).—After Korschelt and Heider.
Another use of the micropylar apparatus noticed by Ayers in the egg of the tree-cricket is that it “serves as a thick, roughened plate, against which the insect may push when ovipositing, without injury to the egg, and without danger that the ovipositor slips from its place.” In Chrysopa eggs the micropyle forms a conspicuous button-like knob, resembling the finely milled head of a certain kind of screw.
Internal structure of the egg.—The egg-contents are surrounded by an outer layer of protoplasm or formative yolk, which is separate from the inner parts of the egg (Fig. 502, do), the latter being mostly composed of the nutritive yolk-element. The superficial protoplasmic layer, called by Weismann Keimhautblastem (K) is, in a few cases, afterwards entirely lost, but in most instances forms a very thin layer of clear protoplasm, slight in extent compared with the yolk-mass within.
The eggs of insects are rich in yolk, only certain eggs, such as those of the Aphides and the egg parasites (Proctotrypidæ) being poor in yolk. The eggs of heterometabolous insects have been said by Brauer to contain relatively more yolk than those of the Metabola, particularly the Diptera; though, as Wheeler observes, this rule has some exceptions, the eggs of the 17–year Cicada being very numerous and small.
This he thinks is a greater advantage to the insect than the production of a few large eggs, “when we consider the extremely long period of larval life and the vicissitudes to which the larvæ may be subjected during all this time.” “Similarly, Meloë angusticollis produces a large number of very small eggs, while the eggs of the smaller beetles (Doryphora, e.g.) are much larger. But Meloë is a parasitic form, and probably only a few of its many offspring ever succeed in gaining access to the egg of the bee.”
In the eggs of Chrysopa the yolk-granules are remarkably small, so that the primitive band is in strong contrast to the yolk in color and density. When crushed, the yolk does not flow out as a liquid, but in a pasty mass, and we have questioned whether, as in the eggs of Limulus, whose yolk is solid with fine granules, the denseness of the yolk is not connected in the way of cause and effect with their exposed situation.
The central or yolk-mass (Fig. 502, do) consists chiefly of rounded masses of yolk, with fat-globules, between which extends a fine network of protoplasm.
The elements of the yolk are spherical and strongly refractive, by pressure becoming polygonal structureless homogeneous bodies.
The germinal vesicle of the ripe insect-egg lies in the centre of the yolk, where it appears as a large vesicle-like cell-nucleus containing a few chromatin elements.
Before the eggs of animals can be fertilized, they require in some observed cases, and probably in animals in general, to undergo a series of changes, which, as observed in the starfish, etc., consists in the replacement of the germinal vesicle by a very much smaller egg-nucleus, and also at the same time the construction at one pole of the egg of the directive or polar bodies (Fig. 502, r). Towards the end of the ripening process of the insect egg this vesicle, according to Blochmann, passes to the dorsal surface of the egg, and is transformed into the directive spindles (Richtungspindel).
The egg next requires the penetration and admission into the yolk-interior of a spermatozoön.
This process is essentially in insects, as in other animals, the fusion of the sperm-nucleus with the nucleus of the egg. Under normal conditions but a single spermatozoön is required for fertilization. As shown by Hertwig, in the sea-urchin, after the spermatozoön has penetrated into the egg, the head, and the small rounded body, called a centrosome, can still be recognized, but the tail becomes fused with the yolk of the egg. In the protoplasm of the egg (called cytoplasm) the achromatic end of the sperm-nucleus gives rise to conspicuous rays, like those observed in ordinary cell-division. Preceded by these rays, the sperm-nucleus or male pronucleus (Fig. 502, p) moves towards the nucleus of the egg, and finally fuses with it, thus forming a new single nucleus. This latter, which is called “the cleavage nucleus,” rapidly forms a nuclear or “cleavage spindle” (Fig. 503). This act gives an impulse to the cleavage of the egg, which is the first step in the formation of the embryo. All these changes have yet to be worked out in detail in insects by microscopic sections of the egg, whose generally hard and opaque egg-shells present great obstacles to such work.
In insects as in most other Arthropoda the segmentation of the yolk is superficial and not total. The ovum is centrolicithal, i.e. the yolk is concentrated at the centre of the egg, and surrounded by a peripheral layer of transparent protoplasm (the Keimhautblastem).
Fig. 504.—Formation of the blastoderm of Pieris cratægi: A, longitudinal section through the egg, with two masses of protoplasm in the yolk. B, a blastoderm-cell at the upper end. C, a later stage, with more blastoderm-cells.—After Bobretsky.
The first step in segmentation is the movement of the first division-nucleus (i.e. that in the fertilized egg arising from the union of the sperm-nucleus with the female pronucleus) towards the interior of the egg in order to multiply itself by the mode of indirect nuclear division (Figs. 504, A, and 507).
Fig. 505.—Embryology of the mole-cricket: 1, egg in which the amœboid nuclei (abc) are moving toward the surface; 2, egg in which the nuclei (abc) have reached the surface, and show an active nucleus-formation; 3, the blastoderm-cells have no nucleus, and are placed at equal distances apart; 4, the blastoderm-cells now forming a continuous layer; 5, cross-section of the egg with blastodermic disk, also showing the disposition of the endodermal cells; 6, cross-section of the blastodermic disk, with the myoblast cells (mb) already formed; 7, cross-section through the thorax of the embryo, the body-cavity extended into the limbs.
Fig. 505 concluded.—Later stages in the embryology of the mole-cricket: 8, longitudinal section of the embryo; the yolk-pyramids (yp) form a common inner yolk-mass (y). 9, section through the heart; H, cavity of the heart; the two halves of the heart-sinuses having united dorsally, ventrally they are still open and are bounded by the walls of the mesenteron. 10, cross-section of an embryo, showing the blood-lacunæ separated on the back by the dorsal organ (do); the intestinal fasciated layer (Darmfaserblatt) has not completely enclosed the yolk. 11, embryo completely segmented, with the rudiments of the appendages, labrum (lab), and nervous ganglia (pc-ng). 12, a more advanced embryo, showing the stomodæum (st) indicated as a frontal protuberance. 13, section through the recently hatched larva, showing the cells of the mesenteron or chyle-stomach, and the cellular layer on the front surface, also the proventriculus or crop.
The origin of numerous division-nuclei as the offspring of the first has been observed to take place in the eggs of those insects (Aphides, Cecidomyia, and Cynips) which have a slight amount of yolk. Yet in the large, ordinary eggs of insects with an abundance of yolk there is no doubt, say Korschelt and Heider, that these numerous division-nuclei, which soon after the process of oviposition are scattered within the egg between the yolk-spheres, and are enveloped by a star-shaped protoplasmic layer, and which constitute the formative elements of the blastoderm,—there is no doubt but that they have practically arisen through indirect nuclear division from the first division-nucleus.
The process of formation of the blastoderm in ordinary eggs with abundant yolk was first observed by Bobretsky in the eggs of a moth (Porthesia) and Pieris, also by Graber, and more recently by Blochmann in Musca, and by Heider in Hydrophilus.
In the earliest stage observed by Bobretsky there first appear after fertilization a few (the smallest number four) cell-like, minute amœboid masses of protoplasm, each with a distinct nucleus. A few (one at least) of these bodies gradually pass out of the centre of the yolk to the surface of the egg (Fig. 504, A, n), these becoming larger and rounder, and from one or two of these nuclei (B, bc) the blastoderm originates (C, bl). Those nuclei remaining in the yolk increase in number and afterwards become the nuclei of rounded masses of yolk-granules, forming the so-called yolk-spheres which Bobretsky regards as true cells.
To the few blastoderm cells situated on the upper end of the egg are added others which continue to pass from the yolk to the periphery, and then the blastoderm spreads out farther and farther from the upper end of the egg until finally it covers or envelops the whole yolk. This layer of cells is called the blastoderm.
As to the origin of the primitive amœboid cells, Bobretsky is in doubt, but is disposed to think that they are the result of the subdivision of the germinative vesicle or nucleus of the ovarian egg-cell. In this connection may be quoted the observations of Graber, who states that an examination of the ovarian cell at an early period has revealed the presence, in the centre of the yolk, of a number of amœboid cells, which appear to have been formed by the division of the germinal vesicle. These “primary embryonic cells” have a relatively large nucleus and a number of nucleoli. Several may be seen to unite with one another by means of their pseudopodia, and they may also be observed to undergo division. With this account may be compared the results obtained by Korotneff in his work on the embryology of the mole-cricket (Fig. 505).
Fig. 506.—Four successive stages in the formation of the blastoderm of Calliphora vomitoria (the figures represent segments of cross-sections through the fly’s egg): A, the nuclei of the division-cells have arranged themselves parallel with the outer surface of the egg. B, the division-cells fused with the “keimhautblastem.” C, the outer surface becomes furrowed by indentations; all the nuclei of the blastoderm-cells in process of division. D, the blastoderm-cells form a high cylinder-epithelium: b, “keimhautblastem”; bz, blastoderm-cells; d, nutritive yolk; dz, yolk-cell; fz, so-called division-cell; i, inner “keimhautblastem.”—After Blochmann, from Korschelt and Heider.
The result of these and of later observations, especially those of Blochmann on Musca, and those of Heider on Hydrophilus, show that the division-nuclei lie near the centre of the egg, along the longitudinal axis (Fig. 507, A). Each of these nuclei is enveloped by a star-shaped mass of protoplasm, and on the whole resembles a wandering amœboid cell. These isolated masses of protoplasm are all connected by a fine network of rays, which unite to form within the yolk a syncytium. Afterwards, in the later stages, these division-cells, as they may be, though somewhat incorrectly, regarded, move nearer the periphery and arrange themselves into a plane parallel with the surface (Figs. 506, A, 507, B). Continuing to divide, they reach the surface and fuse with the peripheral protoplasmic layer (Figs. 506, B, 507, C). Then follows the division into single cell-territories (Figs. 506, B, 507, C), corresponding to the division-nuclei, through the appearance of furrows which pass in from the outer surfaces of the egg into the interior and gradually penetrate the entire “keimhautblastem.” In this way the surface of the egg is covered with an epithelium (blastoderm). In many insects the so-called inner “keimhautblastem” (Fig. 506, D, i) is formed by the separation of a layer of protoplasm which contains larger granules and are accumulated between the blastoderm and the upper surface of the central nutritive yolk-mass. By the addition of this plasmic layer the cells of the blastoderm increase in height, and now form a cubical or cylinder epithelium, which continuously envelops the surface of the egg. (Korschelt and Heider.)
Fig. 507.—Formation of the blastoderm in Hydrophilus: b, completed blastoderm; d, yolk; f, so-called division-cells; k, “keimhautblastem”; z, yolk-cells.—After Heider, from Korschelt and Heider.
The embryo first arises as a whitish streak or band-like thickening on the ventral side of the egg, and is variously called the “primitive streak,” “primitive band,” “germinal band,” or “embryonal streak.” In most cases the primitive band is divided at regular intervals by transverse furrows, indicating the limits of what are to be the body segments.
Cross-sections (Fig. 509) show that the band is composed of several layers, i.e. an outer layer (ectoderm) and an inner layer which comprises the endoderm and mesoderm, and so long as these two layers are not sharply differentiated from one another, this second layer may be called, with Kowalevsky, “the inner lower layer, or ento-mesoderm” (Figs. 508, 509, B, C, u).
It is characteristic of insects, only rarely occurring in other arthropods (e.g. the scorpion), that the primitive streak is not situated on the surface of the egg, but becomes overgrown by a folded structure (Fig. 508, af) rising from its edges, the amnion-fold, so that it appears somewhat depressed or sunken in under the upper surface of the yolk. While the amnion-folds are extending from all sides over the primitive band, there becomes formed under it, by the invagination of the outer surface of the egg, a cavity, the amnion-cavity (ah), which, when the amnion-fold has completely overgrown the primitive band and united together (Fig. 509, C), appears completely closed from without.
Fig. 508.—Two schematic median sections through an insect-embryo to represent the development of the embryonal membranes. In A the primitive streak is not wholly overgrown by the amnion-fold. In B the amnion-folds have united with each other and completely overgrown the primitive streak: a, fore, b, hind, egg-pole; v, ventral side; d, dorsal side; af, amnion-folds; ah, amnion-cavity; am, amnion; do, yolk; ec, ectoderm; k, head-end, k′, hinder-end, of the primitive streak; s, the part of the serosa arising from the amnion-fold; s′, the part of the serosa arising from the unaltered blastoderm; u, lower layer.—After Korschelt and Heider.
Formation of the embryonic membranes.—The amnion-folds finally completely overgrow the primitive band (Fig. 509, B and C), and form the embryonal membranes. The primitive band is seen after its completion to be overgrown by a double cellular epithelial membrane. The outer of these two membranes, that which arises from the outer leaf or layer of the amnion-fold, is the serosa (Figs. 508, B; 509, C, s; 510). This passes continuously into the unchanged part of the blastoderm, which has no part in the formation of the primitive band and germ-layers, and which covers the outer surface of the yolk. Thus the serosa, which is usually held to include this portion also of the blastoderm, forms a closed sac which covers the whole surface of the egg, with one part extending over the surface of the yolk, and the other over the primitive band (Fig. 510).
Fig. 509.—Diagrammatic cross-section through three successive stages of the primitive streak, and growing embryonal membranes of insect-embryos. A, formation of the ventral plate and of the gastrula invagination (g). B, upward growth of the amnion-folds (af). C, complete overgrowth of the primitive band through the amnion-folds: v, ventral side; d, dorsal side; af, amnion-folds; ah, amnion-cavity; am, amnion; bl, blastoderm; bp, ventral plate; do, yolk; ec, ectoderm; s, serosa; u, under or inner layer.—After Korschelt and Heider.
The inner of the two layers, called the amnion (Fig. 509, am), is more closely connected with the embryo. The amnion and ectoderm of the primitive band together form a completely closed sac, whose lumen forms the amniotic cavity. Originally connected with the serous membrane, it splits off from the primitive band about the time the appendages begin to bud out, and continues to closely envelop the body and appendages, as seen in Fig. 509. Both of these membranes are, before the time of hatching, either absorbed, or, as in Lepidoptera, retained. The amnion is retained until after hatching in the locust, etc. In certain Coleoptera the serosa is retained, and the amnion is absorbed (Fig. 532), while in Chironomus and the Trichoptera the serosa is absorbed, and the amnion retained, with the egg-shell or chorion. Hence we have eight layers in the winged insects[81] during embryonic life: