1919] McCulloch: Life Cycle of Crithidia and Trypanosoma 177 



final spores the characteristics of these rectal forms became clear and 

 it was relatively easy to find them in both the mid-stomach and pyloric 

 expansion. In the "crop" of the lupine bug the multiple fission 

 gives rise to numerous small zooids. Some few of these zooids, on 

 reaching a certain stage of development, become the well protected 

 final spore forms. Their development into mature flagellates is ar- 

 rested for an indefinite time, perhaps in response to some unfavorable 

 internal or external chemical condition. The periplast becomes thick- 

 ened and these zooids stain deeply and retain the stain much better 

 than the unprotected zooids. Since only a few of the zooids be- 

 come thus encysted the change cannot be regarded as a general 

 change in response to some general stimulus. In the infections of 

 the mid-stomach and pyloric expansion some of the final spore forms 

 can be found at almost any period in the life history of the flagellates. 

 Considering that these are the only stage of the life cycle of the 

 flagellate which have been found in the rectum they have been re- 

 garded as the final spore forms, which upon being ingested with food 

 by another insect host become the initial infective spores. Our observa- 

 tions upon these cannot be regarded as conclusive as yet, owing to 

 the fact that experimentally we have not produced infection with 

 these spores. 



THE DEGENERATIVE SERIES 



The degenerative series includes all individuals of the life cycle 

 other than the final spore forms and their antecedents which have 

 just been discussed. It has already been pointed out that the rectal 

 phase of C. euryophthalmi in the pyloric expansion of the lupine bug 

 is comparable to the rectal phase of T. lewisi in the rectum of the 

 flea. In the lupine bug the degenerating forms abound in the poste- 

 rior portion of the pyloric expansion and their number is apparently 

 not decreased by constant elimination under normal conditions of 

 such parasites from the intestinal contents, which pass through a 

 relatively long intestine before reaching the colon and rectum. If 

 any flagellates succeed in passing out with the intestinal matter into 

 this long intestine they are evidently destroyed by the new chemical 

 medium before they reach the rectum. In the flea the degenerating 

 forms are in the rectum and it is possible that their number is 

 repeatedly being decreased with each discharge of the feces. 



There is little danger of confusing the developmental and degenera- 

 tive series in the life-cycle of C. euryopkthalmi since the differentia- 



