Observations on the Life-Cycle of Helkesimastix faecicola. 369 



three serious objections to this latter view. Briefly stated, these are as 

 follows : — (1) Trypanosomes can be successfully inoculated, without limit, 

 into fresh vertebrate hosts, i.e. the same " constant " physiological medium, 

 without showing (so far as is known) conjugation. (2) Binucleate flagellates 

 of insects alone (e.g. certain leptomonads) have no alternation of hosts, 

 and syngamy is apparently just as little likely to be found in these 

 parasites as in the trypanosomes themselves. (We are inclined to think, 

 indeed, that syngamy may have been lost in the ancestral trypanosome form, 

 before ever it acquired an alternation of hosts.) Lastly, we have the 

 important case of the Htemosporidia, intracellular parasites which all have 

 the change of hosts, and in all of which conjugation regularly occurs. Why 

 should not the physiological stimulus afforded by the change of environment 

 have influenced them also ? And, on the other hand, there is no reason to 

 doubt that the primitive ancestors of the trypanosomes underwent a pro- 

 cess of syngamy, since it appears likely that many of these lowly proto- 

 monadine forms, among which the origin of the trypanosomes is to be sought, 

 possess this feature. 



We consider, therefore, developing the ideas expressed by Cropper and Drew 

 (loc. eit.) along the line indicated by the experimental facts adduced above, that 

 the loss of syngamy is due to the surfeit of nutrition, together with the non- 

 toxicity of the medium, that is to say, the absence (in excess) of the chemical 

 substance or substances to which the flagellates react normally by the 

 cessation of multiplication and the onset of conjugation. It will be readily 

 apparent, from what has been just pointed out, how these factors have pre- 

 vailed in the case of the trypanosomes. And we think that a similar 

 explanation can be applied, not only to the case of insectan Binucleata, but 

 probably to that of many other parasitic flagellates as well. 



vol. lxxxviii. — B. 



