1919] McCulloch: Life Cycle of Crithidia and Trypanosoma 169 



completely destroyed within a certain radius of the parasites. In 

 the latter the small forms are scattered in the cytoplasm of the host- 

 cell. At d there is still another group of elongate flagellates, which 

 are probably the results of another process of multiple fission. They 

 are approximately the same size and shape, and are in the same stage of 

 development. Whether they are merozoites from a somatella or zooids 

 from endogenous buds is not clear, but they have broken apart some- 

 what and are making their way to the periphery of the host-cell. At / 

 a flagellate is protruding through the cell wall and the posterior end 

 is directed forward to penetrate the tissue. In the study of the living 

 material of both C. leptocoridis and C. euryophthalmi crithidias have 

 been observed to direct their posterior ends forward and to use the 

 flagellated ends as propellers in penetrating tissues. . 



The early stages of the intracellular multiple fission have not yet 

 been found within the epithelial cells of the crop and the more ad- 

 vanced stages, such as shown in figure 39, do not indicate definitely the 

 method of multiple fission. These intracellular crithidias had been 

 described as the results of multiple fission within somatellas before 

 the discovery of the stages of the endogenous process of multiple 

 fission in this life cycle. The evidence is not absolutely convincing 

 either way, but there is at the present state of the investigation a 

 preponderance of evidence in favor of their probable origin by the 

 plasmotomy of a somatella. The circular outlines of the vacuoles in 

 which the groups of parasites are found in figure 39 suggest the 

 formation of the spheroidal somatella. Further there are no evi- 

 dences of discarded flagella within these cells which suggests the possi- 

 bility that endogenous budding might have occurred with the forma- 

 tion of circular cavities in the cytoplasm of the host-cell. On the 

 other hand, if the numerous small oval spores are due to a process of 

 multiple fission within a somatella the subsequent plasmotomy has 

 taken place very early. Usually the breaking apart of the merozoites 

 does not occur until they have become elongate flagellates. Moreover, 

 the exact method of the process of intracellular multiple fission is 

 not so important as the fact that under certain conditions crithidias 

 become intracellular and destroy the epithelial lining of the digestive 

 tract which they penetrate. Each destruction of a host-cell thus also 

 means a tremendous increase apparently in the number of the para- 

 sites. The intracellular crithidias are not found frequently in the 

 preparations. We have no proof that it is an obligatory phase, though 

 it might well be so. Nor have we evidence that it follows the forma- 



