THE H^MOFLAGELLATES. 199 



Large female forms laden with reserve materials are unable 

 to pass through the proboscis of the gnat. Young females, 

 on arriving in the blood, at once penetrate into the erythro- 

 cytes. Growth is much slower than in the case of the in- 

 different forms. Moreover, the parasites change host-cells 

 less frequently, and the older ones appear to be no longer 

 able to assume the Trypanomorpha-form. Such indivi- 

 duals leave one red corpuscle, wander about in the plasma, 

 and then pass into another corpuscle while in the gregariui- 

 form condition. A ripe adult megagametocyte is incapable 

 of movement, and remains enclosed by the now pallid and 

 disorganised host-cell, the nucleus of which has been pushed 

 to one side. Its general structure is already well-known 

 through the older researches on Halteridium. Schaudinn 

 points out that, in contact with the relatively small nucleus 

 (trophonucleus), there can be seen a correspondingly small 

 kinetonucleus. 



As in the gnat the female Trypanosomes are the only 

 forms able to survive unfavourable circumstances, so here, in 

 the blood of the bird, the megagametocytes alone remain 

 when the indifferent Trypanosomes and microgametocytes 

 have all died off. Similarly, they are able to cause, at 

 intervals, a recurrence of the infection by undergoing the 

 process of parthenogenetic development above described, in 

 the same way that the recurrence of malaria is brought about 

 in the case of Plasmodium (see Schaudinn [97]). 



(4) Maturation of the Megagametocyte, and Ferti- 

 lisation of the Megagamete by the Microgamete. — 

 Maturation and fertilisation do not take place until the 

 sexual forms are transferred, Avith the blood, to the ali- 

 mentary canal of the gnat. The main outlines of the process 

 have been well described by MacCallum (72) for another 

 species of Halteridium, so that our present author directs 

 attention more especially to the cytological details. 



As soon as the megagametocyte leaves the warm-blooded 

 host it becomes rounded off, ruptures the delicate envelope of 

 the host-cell still surrounding it, and is thus set free. The 



