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 Robertson[63] (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.
Fig. 30.—Trypanosoma gambiense. Development in the fly, Glossina palpalis. a, slender, proventricular form; b, multinucleate form; c, d, crithidial forms; e, infective type of trypanosome found in salivary gland. × 2,500. (After Robertson.)
Invasion of the Salivary Glands of the Fly.—Long, slender trypanosomes from the proventriculus pass forward into the hypopharynx. They then pass back along the salivary ducts, about sixteen to thirty days after the fly’s feed. The trypanosomes reach the salivary glands as long, slender forms. In the glands they become shorter and broader, attach themselves to the surrounding structures, and assume the crithidial facies (fig. 30, c, d). As crithidial forms they remain attached to the wall and multiply in the glands. These crithidial stages differentiate into the short, broad trypanosome forms, capable of swimming freely (fig. 30, e).
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.