136 TJnivi rsity of California Publicatioiis in Zoology. ( v °>- 6 



division similar to that described above for the indifferent forms, 

 and as a result are more abundant than any other in the later 

 stages of infection. We have been unable to decide whether the 

 trophonucleus extrudes chromatin, as in the female forms, or not ; 

 but the intranuclear centrosome often exhibits budding (PI. 17, 

 figs. 38-40). The masses which thus arise simply disappear. 



Mitosis is observed more distinctly in this elongated tropho- 

 nucleus than in any other, the equatorial plate being very dis- 

 tinct, and the two centrosomes widely separated, with a series of 

 very fine fibres extending between them. 



The long, slender body of this extreme type of dimorphon, 

 and absence of granules, together with its well-developed flagel- 

 luin and elongated nucleus, and also comparison with other forms 

 of trypanosomes, supports the view that it is to be regarded as 

 the male form. 



The life-cycle of T. dimorphon within the vertebrate host may 

 therefore be briefly summarized in the accompanying diagram. 



SUMMARY. 



In Trypanosoma dimorphon there are three distinct types 

 which may be regarded as indifferent, male, and female forms, 

 respectively. 



The indifferent trypanosomes, from which the other two types 

 originate, are especially characterized in this species by possess- 

 ing a complete asexual cycle (cyst formation) that goes on in 

 the vertebrate host. This cycle is very similar to that of 

 Crithidia described by 1'atton (1909) and other authors. In at 

 least one other species of trypanosome the c.vst formation occurs 

 in the alimentary canal of the blood-sucking invertebrate thai 

 serves as the intermediate host (Robertson, 190!)). 



In addition to this cycle, some of the indifferent forms may 

 develop further along two lines, culminating in the "stumpy" 

 and "long" forms described above, which perhaps may be female 

 and male respectively. The evidence for regarding them as such 

 is inconclusive since we have never observed conjugation; prob- 

 ably it takes place in the intermediate host. The "stumpy" 

 forms certainly seem to correspond with the short female gametes 



