The undulating membrane is a lateral extension of the ectoplasm or periplast, and is the main agent in locomotion. It is edged by the flagellum, which forms a deeply stainable border to it. Within the membrane substance, often arranged parallel with its edge, are a number of fine contractile elements, the myonemes. These contractile elements may also occur on the body of the trypanosome. They are easily seen in some large trypanosomes, but are difficult of demonstration in others, owing to their great fineness.

Multiplication of trypanosomes in the blood is brought about by binary longitudinal fission (fig. 26). Division is initiated by that of the blepharoplast and nucleus. The division may be equal or subequal, whereby differences in size of individuals partly arise. Multiple division by repeated binary fission, without complete separation of the daughter forms, is known in some trypanosomes (e.g., T. lewisi), and rosettes of parasites thereby are produced (fig. 27).

The classification of trypanosomes is very difficult. Laveran (1911)54 has suggested the examination of the relative length of the flagellum as a diagnostic character, and so arranged these flagellates in mammals in three groups. The first group included those trypanosomes always having part of the flagellum free (e.g., T. evansi, T. vivax); the second group comprised forms without a part of the flagellum free (e.g., T. congolense), while the third group included forms some members of which have free flagella, while others have not (e.g., T. gambiense). Bruce55 (1914) and Yorke and Blacklock56 (1914) have also devised classifications.

Resting stages of some trypanosomes have been found in the internal organs of their vertebrate hosts. The formation of these oval, Leishmania-like bodies will be noted in individual cases later. Similar small oval bodies form an important phase in the life-history of T. cruzi, which multiplies normally by multiple fission or schizogony into these oval, daughter elements, and not by binary longitudinal fission in the circulating blood.

Polymorphism in trypanosomes (e.g., T. gambiense, T. rhodesiense) is now interpreted as a phenomenon resulting from growth and division.57 Long, thin forms are those about to divide. Fully mature forms are shorter and broader. Various intermediate types occur and represent growth forms. Formerly, polymorphism was interpreted in terms of sex, thin forms being regarded as males, broad forms as females, while the intermediate types were termed indifferent. Conjugation was not observed, and there is no evidence in support of the sexual interpretation.

The transmission of trypanosomes from one vertebrate host to another is usually accomplished by the intermediation of some biting arthropod in the case of terrestrial animals, while leeches are usually considered to act as transmitters in the case of the trypanosomes occurring in aquatic animals. Developmental phases of the life-histories of trypanosomes occur in the invertebrate transmitters, and will be considered in individual cases.

Trypanosoma gambiense, Dutton, 1902.

Syn.: Trypanosoma hominis, Manson, 1903. Trypanosoma nepveui, Sambon, 1903. Trypanosoma castellanii, Kruse, 1903. Trypanosoma ugandense, Castellani, 1903. Trypanosoma fordii, Maxwell Adams.

In vertebrate blood Trypanosoma gambiense is polymorphic, for long, thin forms may be seen in contrast with short, stumpy forms, as well as intermediate forms (fig. 29, a—c). This polymorphism has been interpreted in terms of sex, especially by German investigators, following Schaudinn (see above). However, there is no evidence of conjugation, and the polymorphic forms are more easily interpreted in terms of growth and division, for the long thin forms are potential dividing organisms, and the stumpy or short parasites, with little or no free flagellum, are the adult individuals.

Morphology of T. gambiense in the Circulating Blood.

T. gambiense varies from 13 µ to 36 µ in length, its average length being 24·8 µ, as was determined in 1913 by exact biometrical methods by Stephens and Fantham.58 Three forms of parasite occur. According to Miss Robertson,59 the relatively short forms from 13 µ to 21 µ long may be regarded as the mature or “adult” type of parasite in the blood. They carry on the cycle in the vertebrate. From them intermediate forms, which are longer than the “adult” but at first have the same breadth, arise by growth. They possess a free flagellum. The intermediate forms grow into long individuals, which are those about to divide. The products of division give rise, directly or indirectly, to the adult forms.

The organism has an elongate body with an anterior or flagellar end and a blunter posterior or non-flagellar end. The protoplasm is finely granular, large inclusions being rare. The central nucleus is oval and large, often containing most of its chromatin concentrated as a karyosome, with small granules only scattered near or on the fine nuclear membrane. The blepharoplast is either rounded or rod-shaped. The undulating membrane is thrown into folds and is bordered by the flagellum. A small basal granule may be present near, or at the actual origin of the flagellum.

Multiplication in the vertebrate is brought about by longitudinal division. According to the recent account of division by Miss Robertson, the blepharoplast doubles, then the flagellum splits for the greater part of its length, and the daughter flagella separate, one being shorter than the parent flagellum. The nucleus often shows two well marked dark granules on the membrane at opposite poles, and these appear to act as centrosomes. Nuclear constriction occurs and the halves gradually separate. Finally the two daughter organisms become free, the aflagellar end splitting last. The products of division may be equal or unequal. Repeated division goes on in the general circulation until the blood swarms with parasites. Then the trypanosomes gradually disappear, and a period occurs when it is practically impossible to demonstrate the parasite in the blood. At such a period, trypanosomes can be obtained by puncture of the enlarged lymphatic glands or of the spinal canal, or can be found in the internal organs, more particularly in the spleen, lungs, liver and bone-marrow. In the latter organs, latent bodies are produced (fig. 29, d—f) which are capable of again becoming flagellates and entering the general circulation. Their formation was described by Fantham (1911).60 The parasite contracts, the blepharoplast migrates towards the nucleus, a very thin coat differentiates around the two nuclei and a certain amount of cytoplasm, and the parts exterior to the coat disintegrate, leaving a small, oval body behind. Fuller details are given in connection with T. rhodesiense. Laveran (1911)61 considers that latent bodies are “involution” forms, but acknowledges that they can flagellate and become infective in fresh blood.

No multiplication of the trypanosomes within the cells of the lung, liver or spleen of infected monkeys was found by Miss Robertson in her recent researches.

There appear to be negative periods in infected monkeys, since, although trypanosomes may occur in their blood at such times, they are not infective to Glossina.

Development in Glossina palpalis.—The principal accounts are those by Sir D. Bruce and his colleagues (1911),62 and by Miss Robertson63 (1912), whose results will be followed. According to the latter investigator T. gambiense never enters the body cells of the fly (G. palpalis), nor does it penetrate the gut wall into the body cavity. Practically no crithidial stage occurs in the fly’s main gut, but a trypanosome facies is retained therein.

After the trypanosomes are ingested by the fly during a meal of infected blood, sooner or later multiplication occurs. This development usually begins in the middle or posterior part of the mid gut, and trypanosomes of varying sizes are produced. After the tenth or twelfth day, many long, slender trypanosomes (fig. 30, a) are found, which gradually move forwards into the proventriculus. Such long, slender forms represent the limit of development in the lumen of the main gut. The proventricular type, developed about the eighth to the eighteenth or twentieth day, is not infective; it may occur in the crop, but is not to be found permanently there. Between the tenth and the fifteenth days multinucleate forms of trypanosomes are found, and may be styled multiple forms (fig. 30, b). Some of these latter may be degenerative.

Miss Robertson considers the development in the main gut to be indifferent multiplication, and that salivary fluid seems necessary to stimulate trypanosomes to the apparently essential reversion to the crithidial type. The second development in the salivary gland is the essential feature. The short, stumpy forms of trypanosomes (fig. 30, e) finally produced in the salivary glands are alone infective. No conjugation of trypanosomes occurs in the fly. Only about 5 per cent. of captive tsetse flies fed on trypanosome-infected blood become infective, but they probably remain infective for the rest of their lives.

J. G. Thomson and Sinton (1912)64 have obtained in cultures the various trypanosome forms of T. gambiense seen in the fly’s main gut.

Duke (1912)65 found T. gambiense in a species of antelope, the situtunga (Tragelaphus spekei), on Damba Island in Victoria Nyanza. Wild G. palpalis could be infected therefrom. The antelope may then act as a sleeping sickness reservoir in that district, but men are apparently the chief reservoir.

Trypanosoma nigeriense, Macfie, 1913.66

Macfie has recently (August, 1913) described a human trypanosome from the Eket district of Southern Nigeria. It is common in young people. The disease produced does not seem to be of a virulent type in Nigeria, and does not occur in epidemic form. In the early stages the glands of the neck are enlarged. In the later stages—cases of which are rarer—lethargy appears. The parasite is a polymorphic trypanosome, morphologically almost indistinguishable from T. gambiense, though it may be slightly shorter. Macfie recorded the occurrence in his preparations of a few trypanosomes appearing to have a flagellum free during their whole length. Some of the parasites, as seen in a sub-inoculated guinea-pig, are very small (8 µ long). Other trypanosomes have their nuclei displaced somewhat anteriorly. This parasite may only be a variety of T. gambiense. The parasite is perhaps spread by Glossina tachinoides.

Trypanosoma rhodesiense, Stephens and Fantham, 1910.

The parasite was found in the blood of a young Englishman who had contracted sleeping sickness in the Luangwa Valley, North-eastern Rhodesia, in the autumn of 1909. The patient had never been in an area infested with Glossina palpalis.

(1) Morphology.—The morphology of the parasite in man and sub-inoculated rats was studied by Stephens and Fantham in 1910.67 They pointed out a morphological peculiarity in the presence of certain trypanosomes with posterior nuclei in sub-inoculated animals, that is, parasites in which the nucleus (trophonucleus) was situated towards the posterior or aflagellar end, close up to or even beyond the blepharoplast or kinetic nucleus (fig. 31, 4, 5). When the nucleus was beside the blepharoplast, the former was seen to be kidney-shaped (fig. 31, 4). The posterior nuclear forms were of the stout and stumpy variety, and about 6 per cent. of the stumpy forms were found to have their nuclei displaced from the centre. The anterior or flagellar end of these trypanosomes often contained chromatoid granules. T. rhodesiense varies in length from 12 µ to 39 µ68; short stumpy forms vary from 13 µ to 21 µ, intermediate forms from 21 µ to 24 µ, and long, slender forms from 25 µ onwards. The average length is 24·1 µ.

Certain regular periods occur in the course of the trypanosomiasis when few or no flagellate trypanosomes are found in the peripheral blood of the patient or of the sub-inoculated animal. These periods can be explained in terms of morphology, for the trypanosomes are capable of assuming a non-flagellate form in the internal organs of the host, particularly in the lungs and in the spleen. Such forms are known as “latent” or “resting” forms. The term “latent body” was first used by Moore and Breinl in 190769 in connection with T. gambiense. Fantham70 (1911) has described the process of formation of latent from motile forms and the reconversion of the latent bodies into active flagellates. Fresh preparations of splenic blood or lung blood containing trypanosomes were made. A trypanosome gradually withdrew or cast off its flagellum, concentrated its cytoplasm, and became more or less elongate oval. Nucleus and blepharoplast approached one another and came to lie more or less side by side. Then an opaque line often made its appearance around the nuclear area and differentiated as a slight envelope or covering, the cytoplasm external to this merely degenerating. The small, oval, refractile body (fig. 29, d—f) thus formed was a non-flagellate latent body, 2 µ to 4 µ in diameter, like Leishmania or the non-flagellate, multiplicative forms of T. cruzi (fig. 34), and remains temporarily inactive in the internal organs of the host. After this period of inactivity, the non-flagellate body, recuperated by its rest, begins to elongate again. The nuclei separate. From a small vacuole-like portion the flagellum differentiates and forces out the ectoplasm, which assumes the form of the undulating membrane with its flagellar border. Subsequent growth results in the production of the typical trypanosome form, which re-enters the circulating blood and multiplies by longitudinal binary fission. Division of the parasite prior to the formation of a latent body may occur and division of the latent forms themselves is known, though less common. Consequently latent bodies, like the flagellate forms themselves, show diversity in size. The blepharoplast of the latent bodies is sometimes less well marked than in Leishmania (see fig. 29, d-f). Laveran’s views on these bodies have already been given on p. 74.

(2) Animal Reactions.—The posterior nuclear trypanosomes were found in all sub-inoculated animals, such as rats, guinea-pigs, dogs, mice, Macacus, rabbits and horses, but were not seen in the human patient, as few trypanosomes occurred in his peripheral blood. R. Ross and D. Thomson71 found a periodic, cyclical variation in the number of the parasites in the patient’s blood from day to day, the cyclical period being about a week (fig. 32). Fantham and J. G. Thomson72 (1911) found a similar periodic, cyclical variation in the trypanosomes in the blood of sub-inoculated rats, guinea-pigs and rabbits. On counting the parasites in the blood of similar animals inoculated with T. gambiense, they established, by enumerative methods, that T. rhodesiense was more virulent than T. gambiense, while Yorke also showed this marked virulence of T. rhodesiense in practically all laboratory animals. In other words the duration of infection in the case of T. rhodesiense was shorter. It was also found that T. rhodesiense was resistant to atoxyl. The patient, from whom the original strain was obtained, died about nine months after the probable date of infection. Some patients infected with T. rhodesiense have died in an even shorter period, such as four or five months.

In sheep and goats T. rhodesiense causes an acute disease, marked by high fever, œdema of the face, and keratitis, as shown by Bevan and others, death resulting after a relatively short period. T. gambiense gives rise, in these animals, to no symptoms except fever, which may be overlooked. T. rhodesiense produces keratitis in dogs.

Stannus and Yorke (1911) observed T. rhodesiense in animals inoculated from a case of sleeping sickness in Nyasaland. Sir D. Bruce and his colleagues73 have shown (1912) that T. rhodesiense is the parasite usually found in man and in animals sub-inoculated from cases of sleeping sickness in Nyasaland. It has since been found in German East Africa and Portuguese East Africa, while Ellacombe has described a case from North-western Rhodesia.

(3) Serum Reactions.—Interesting experiments on this subject were performed during 1911 and 1912 by various French investigators.

(a) Action of Immune Serum (Mesnil and Ringenbach)74: (1) A goat was infected with T. rhodesiense. Twenty-two days later its serum mixed with T. rhodesiense was injected into a mouse. Result: Protection. (2) The serum mixed with T. gambiense was injected into a mouse. Result: Infection.

(b) Action of Baboon Serum.—Contrary to T. gambiense, T. rhodesiense is very susceptible to human and baboon sera. Mesnil and Ringenbach75 showed that a dose of 1 c.c. of baboon (Papio anubis) serum cured mice infected with T. rhodesiense. In the same dose it acted very feebly on T. gambiense.

(c) Action of Human Serum.—1 c.c. of human serum cured T. rhodesiense mice in three out of four cases; on T. gambiense mice there was no appreciable effect.

Laveran and Nattan-Larrier76 have shown the same, namely, that human sera act on T. rhodesiense, but are quite without action on T. gambiense.

(d) Trypanolytic Reactions.—Mesnil and Ringenbach77 have also shown that the sera of animals (man, monkey and guinea-pig) infected with T. gambiense are trypanolytic for the homologous trypanosome, that is, T. gambiense, but have no action on the heterologous trypanosome, that is, T. rhodesiense.

(4) Cross Immunity Experiments.—(a) Mesnil and Ringenbach78 immunized a monkey (Macacus rhesus) against T. gambiense. It was inoculated with T. rhodesiense on June 7, 1911; on June 27 trypanosomes appeared, the infection being slight; on July 4 it died. A control died in ten and a half days.

(b) Laveran79 immunized a goat and mice against T. gambiense. When they had acquired a solid immunity, they were inoculated with T. rhodesiense. They became infected like the controls.

(c) Laveran and Nattan-Larrier80 immunized a ram against T. brucei, it subsequently became infected with T. rhodesiense.

(d) Laveran81 immunized a ram and a sheep against different strains of T. brucei. Inoculated with T. rhodesiense they both acquired acute infections and died. Conclusion: T. rhodesiense is not T. brucei.

When the converse set of experiments is tried, namely, immunizing an animal against T. rhodesiense, and then inoculating with T. gambiense, the difficulty immediately arises that it is impossible to immunize an animal against T. rhodesiense, owing to its virulence. But a partial and transitory immunity to T. rhodesiense can be obtained by treating the infected animal with drugs, such as arsenophenylglycin. The results, so far as they go, seem to show that an animal immunized against T. rhodesiense is immune not only to T. rhodesiense, but also to T. gambiense, a fact which, according to Mesnil and Léger, does not invalidate the specificity of T. rhodesiense, but tends to show that the two trypanosomes are closely related.

(5) Mode of Transmission and Reservoir.—Kinghorn has shown that T. rhodesiense is transmitted by Glossina morsitans in which it undergoes development. Kinghorn and Yorke82 found that about 16 per cent. of the wild game examined in Northern Rhodesia was naturally infected with T. rhodesiense. The wild game examined included waterbuck, hartebeest, mpala, bushbuck and warthogs. One native dog near the Nyasaland border was found infected, but not domestic stock. Taute doubts whether T. rhodesiense really occurs in wild game. Approximately 3·5 per cent. of the tsetse flies fed on infected animals may become permanently infected with T. rhodesiense, and capable of infecting clean animals. Furthermore, a tsetse fly when once infective probably remains infective for the rest of its life.

Kinghorn and Yorke, however, have shown that climatic conditions, namely, those of temperature, also affect the infectivity of the tsetse fly, as the ratio of flies capable of transmitting T. rhodesiense to those incapable of transmitting the virus is 1 : 534 in hot valley districts (e.g., Nawalia, Luangwa Valley, temperature 75° to 85° F.), while on elevated plateaux (e.g., Ngoa, on the Congo-Zambesi watershed, temperature 60° to 70° F.) the ratio falls to 1 : 1312.

Mechanical transmission by the tsetse fly does not occur, if a period of twenty-four hours has elapsed since the infecting meal.

Developmental Cycle in the Fly.—The period which elapses between the infecting feed of the flies and the date on which they become infective varies from eleven to twenty-five days in the Luangwa Valley, according to Kinghorn and Yorke. Attempts carried out at laboratory temperature on the Congo-Zambesi plateau, during the cold season, to transmit T. rhodesiense by means of G. morsitans were always unsuccessful. The developmental cycle of the trypanosome in the fly is influenced by the temperature to which the flies are subjected (as stated above). The first portion of the developmental cycle proceeds at the lower temperatures (60° to 70° F.), but higher temperatures are necessary for the completion of the development of the trypanosome. Kinghorn and Yorke found that the trypanosomes may persist in the fly, at an incomplete stage of their development, for at least sixty days when the climatic conditions were unfavourable.

The first portion of the developmental cycle of the trypanosome takes place in the gut of the fly. Invasion of the salivary glands of the tsetse is secondary to that of the intestine, but is necessary for the infectivity of the fly. A relatively high mean temperature, 75° to 85° F., is essential for the passage of the trypanosomes into the salivary glands and the completion of their development therein.

Kinghorn and Yorke83 state that the predominant type of trypanosome in the intestine of infected G. morsitans was a large broad form, quite different from that which is most common in the salivary glands. The trypanosome in the glands resembles the short form seen in the blood of the vertebrate host. The authors quoted state that both the intestinal and salivary gland forms of infective G. morsitans are virulent when inoculated into healthy animals.

Bruce and colleagues84 have quite recently (June, 1914) published an account of their investigations of T. rhodesiense in G. morsitans in Nyasaland. (Incidentally it may be remarked that Bruce considers T. rhodesiense to be identical with a polymorphic strain of T. brucei—see pp. 83, 94). The development of T. rhodesiense takes place in the alimentary canal and salivary glands, not in the proboscis, of the tsetse fly. In feeding experiments with laboratory bred flies, as well as with a few wild flies, fed on infected dogs or monkeys, only 8 per cent. of the flies were found to be infected on dissection. Of such infected flies, however, only some allow of the complete development of the trypanosomes within them, in other words only about 1 per cent of the flies become infective. The length of time which elapses before a fly becomes infective varies from fourteen to thirty-one days, averaging twenty-three days, when kept at 84° F. (29° C.). The dominant intestinal type of flagellate in the fly is that seen in the proventriculus, which contains many long, slender trypanosomes. These proventricular forms find their way to the salivary glands, wherein crithidial and encysted forms are seen. They change into “blood forms,” which are short, stumpy trypanosomes and are infective. “The infective type of trypanosome in the salivary glands—corresponding to the final stage of the cycle of development—is similar to the short and stumpy form found in the blood of the vertebrate host.” The cycle is thus very similar to that of T. gambiense in G. palpalis (fig. 30).

Culture.—J. G. Thomson (1912),85 and subsequently Thomson and Sinton, succeeded in cultivating T. rhodesiense in a modified Novy-MacNeal medium. The development obtained resembled that of the trypanosome in the intestine of Glossina.

General Note on Trypanosomes with Posterior Nuclei.

Posteriorly placed nuclei have been found to occur not only in T. rhodesiense by Stephens and Fantham (1910), but also in T. pecaudi by Wenyon (1912), in T. brucei by Blacklock (1912), and in T. equiperdum by Yorke and Blacklock (1912).

Recently Stephens and Blacklock (1913)86 have shown that two trypanosomes, different morphologically, have been confused under the name T. brucei. One of these is polymorphic (i.e., it exhibits long and slender as well as short and stumpy forms) and came from Uganda, while the other is monomorphic and is the original Zululand strain described by Bruce from cattle suffering from “nagana.” Bruce (1914) considers that morphological change has occurred in T. brucei in its passage through laboratory animals, and thus explains the diversity of views. The posterior nuclear forms described by Blacklock occurred in the Uganda strain of T. brucei. (See p. 95.) Similarly, a posterior nuclear form, T. equi, has been separated from T. equiperdum. (See p. 98.)

Again, Bruce and his colleagues on the Royal Society Commission investigating sleeping sickness in Nyasaland, have stated (April, 1913) that “evidence is accumulating that T. rhodesiense and T. brucei (Plimmer and Bradford) are identical.” The exact identity of trypanosomes showing posterior nuclei is, then, far from settled, although Laveran by cross immunity tests has declared that T. brucei is distinct from T. rhodesiense. No one has yet seen posterior nuclei in T. gambiense.

Trypanosoma cruzi, Chagas, 1909.

Syn.: Schizotrypanum cruzi, Chagas, 1909.

The trypanosome was discovered by Chagas87 in the intestine of the bug, Triatoma (Conorhinus) megista, in Brazil, and then in the blood of a small monkey bitten by the bug. A little later it was found in the blood of a child, aged two years, suffering from irregular fever, extreme anæmia and enlarged glands in the State of Minas Geraes, Brazil. Chagas found that he was able to infect many of the usual laboratory animals with the trypanosome, by allowing the bug to bite them. He was also able to culture the parasite on blood agar.

Chagas found the Reduviid bug, Triatoma megista, in the houses of the poorer inhabitants of the Brazilian mining State, and that it attacked the people, more especially the children, at night, biting the face. On this account the insect is called “barbeiro” by the inhabitants. The bite is somewhat painful. The disease has since been found in other parts of Brazil, e.g., Matta de São João in Bahia province, Goyaz, Matto Grosso and São Paulo provinces, as well as in Minas Geraes.

Morphology.—The trypanosome has a large blepharoplast or kinetic nucleus. It is stated to occur both free and in the red blood corpuscles in the peripheral blood. It is about 20 µ long, on an average.

Two forms of the parasite (fig. 33, 6, 7) are described in the human blood. In one free form there is a large egg-shaped blepharoplast and the posterior (aflagellar) end of the parasite is drawn out. The blepharoplast (kinetic nucleus) may have a chromatin appendage. The nucleus is oval or band-like, containing a karyosome. The flagellum, starting close to the blepharoplast or its appendage, has a free portion of variable length. The other free form in the blood has a more or less round, terminal blepharoplast, smaller than in the first form, without a chromatin appendage as a rule. The body of this second form is decidedly broader than that of the first mentioned.

The investigations of Chagas and of Hartmann have revealed two types of multiplication which take place in the internal organs of the vertebrate host.

(a) The first type—which possibly belongs to another organism, Pneumocystis carinii, see p. 90—occurs in the capillaries of the lungs. The flagellate parasite entering the lung capillaries loses its flagellum and undulating membrane. Its body becomes curved, and the two ends fuse, and so an oval mass is formed (fig. 33, 8-11). In some cases the blepharoplast disappears, in other cases it blends or fuses with the nucleus. The nucleus of the rounded parasite then divides into eight by successive divisions (fig. 33, 12-15). Next the body, which is surrounded by its own periplast, also divides, giving rise to eight tiny daughter individuals or merozoites (fig. 33, 15). The merozoites lie inside the periplast, which acts as a sort of “cyst wall.” The merozoites are said to exhibit dimorphism, and Chagas has interpreted the dimorphism in terms of sex. The daughter forms, produced by the parent trypanosomes which kept their blepharoplasts, themselves have blepharoplasts as well as nuclei, and have been termed “males” or “microgametes.” The merozoites, arising from parent trypanosomes which lost their blepharoplasts, have themselves only nuclei, and have been called “females” or “macrogametes.” In the case of the so-called “female” forms the single nucleus divides into two unequal parts, of which the smaller becomes the blepharoplast, and a flagellum is formed later. The so-called “males” possess early a rudiment of a flagellum. Both kinds of merozoites escape from the parent periplast wall, and enter red blood corpuscles. They grow into flagellates within the corpuscles, and then become free as adult trypanosomes in the blood-stream.

Chagas considers this second mode of multiplication to be strictly asexual. By this means the number of parasites in the vertebrate host is increased, and symptoms are produced. On the other hand the first mode of multiplication, seen in the lung capillaries, is considered by Chagas to be a process of gametogony, in which sexual forms are differentiated. He finds that (1) the adult trypanosomes exhibit a dimorphism in human blood rarely seen in artificially infected guinea-pigs. In these guinea-pigs (infected from guinea-pigs) the so-called gametogony in the lungs is seldom seen. (2) The intermediate host, Triatoma (Conorhinus), becomes infective if fed directly on infected human blood, but very rarely so if fed on guinea-pigs. Chagas is led to believe that the occurrence of sexual forms constantly in the blood of man implies a greater resistance to infection on the part of man than on the part of guinea-pigs or other animals, assuming the general hypothesis that the formation of gametes represents a reaction of the Protozoön to unfavourable conditions. In human infection the number of parasites is always less than in laboratory animals, and their presence in the blood is transitory, lasting from fifteen to thirty days in acute cases. In many cases examination of the tissues at death has shown the presence of parasites in patients who did not exhibit them in the general circulation.

Life History in the Invertebrate Host.—About six hours after the ingestion of infected blood by the bug (Triatoma megista), the kinetic nucleus of the trypanosome moves towards the nucleus, and the flagellum is usually lost (fig. 35, 1-5). The parasite becomes rounded and Leishmania-like (fig. 35, 3-5), and multiplies rapidly by division. After a time, multiplication having ceased, the rounded forms become pear-shaped and develop a flagellum at the more pointed end. Crithidial forms (fig. 35, 7) are thus produced and pass into the intestine, where they multiply and may be seen in about twenty-five hours after the ingestion of blood. The crithidial forms may also be found in the rectum and fæces. The last stage in the invertebrate is a small, trypanosome-like type, long and thin with a band-like nucleus and conspicuous kinetic nucleus. These parasites are found in the hind gut and in the body cavity. They find their way into the salivary glands, and are the forms (fig. 36) which are transmissible to a new vertebrate host. The development in the bug takes about eight days altogether, after which time the bugs are infective.

There are thus three principal phases in the development of T. cruzi in Triatoma megista: (1) A multiplicative phase (Leishmania-like) in the stomach of the bug, (2) a crithidial phase, which is also multiplicative, in the hind-gut, and (3) a trypanosome phase, which is “propagative,” and apparently passes through the wall of the alimentary canal into the body cavity and so into the salivary glands.