384 PROCEEDINGS OF THE ACADEMY OF [June, 



In so far as the first of these stages is concerned, that is, the evolu- 

 tion of the microgametes and macrogametes, the parallel between 

 sarcosporidian and coccidiomorphan is exact. 



In the second stage, the gaps in our knowledge of the course of 

 events in the Sarcosporidia prevents as precise a comparison. We do 

 not know what happens between the time the parasite leaves the 

 epithelium and the time it invades the muscle, nor has the stage 

 which appears in the feces ever been seen. Nevertheless, the course 

 of events in the two cases must be more or less similar. For in both, 

 the zygote divides into a number of small elements which serve to 

 carry the infection either to other hosts or to other parts of the same 

 host. In the Coccidiomorpha, there is typically a first division 

 into spores, the protoplasm of which secondarily divides into sporo- 

 zoites. In the sarcosporidian, it is in evidence that the zygote 

 divides into small elements, the further history of which is not 

 known. But in any event, it must be some product of the zygote 

 which on the one hand invades the muscles and on the other develops 

 into the encysted fecal stage. It is conceivable that it is an encysted 

 zygote which infects the feces, but we do not know. 



In the third stage, the course of events differs in detail in the two 

 groups of animals, but the end results are the same, since in both 

 there are produced a large number of elements destined to evolve 

 into the sexual stages. In the Coccidiomorpha, there is extensive 

 growth, followed by multiple division, this cycle being repeated an 

 indefinite number of times. In the Sarcosporidia, there is an indefin- 

 ite number of bi-partitions alternating with only enough growth to 

 restore the element to its original size. In the one case, the Sar- 

 cosporidia, multiplicative energy is continuous, while in the other, 

 the Coccidiomorpha, it is periodic, but this difference is not essential. 



We may next endeavor to homologize the several stages in the life 

 history of Sarcocystis muris with those of the Coccidiomorpha. In 

 the first place, it is entirely evident that the sarcosporidian "spore" 

 is the homologue of the coccidiomorphan merozoite. The sexual 

 stages are alike. The multiplication products of the sarcosporidian 

 zygote are presumably the homologues of either the spores or sporo- 

 zoites of the Coccidiomorpha. The sporoblast is not so easy to place. 

 It may correspond to either the sporozoite or the trophozoite of the 

 Coccidiomorpha. A knowledge of its derivation is necessary before 

 this point can be determined. Since, however, the sporoblasts are 

 all of much the same size, whether occurring singly or within groups, 

 the distinction here between sporozoite and trophozoite would be 

 largely a matter of terms. 



