PROCEEDINGS OF THE ACADEMY OF [Jan., 



THE SEXUAL EVOLUTION OF SARCOCYSTIS MUEIS. 

 BY HOWARD CRAWLEY. 



A number of years ago, at the University of Pennsylvania, an 

 investigation of the early stages of the evolution of Sarcocystis maris 

 in the intestinal cells of the mouse was undertaken. This investiga- 

 tion was interrupted for several years, but was resumed at the 

 laboratory of the Zoological Division of the Bureau of Animal 

 Industry, Washington, D. C, and the first definite results obtained 

 were outlined in a preliminary note published in 1914 (Crawley, 1914). 



In this note it was shown that following ingestion of the so-called 

 spores of this parasite, penetration of the intestinal cells of the mouse 

 was effected within about two hours. Once within the cells the 

 spores rapidly underwent profound changes and after the lapse of 

 about nine hours they had separated into two categories, which were 

 interpreted to represent males and females. In the case of the 

 supposed males, development took the form of a loss of most if not 

 all of the cytoplasm, so that the parasite became reduced to a nucleus . 

 which, however, was of considerably larger size than that of the 

 original spore. Later, the chromatin of this nucleus became col- 

 lected into a number of small rounded masses placed at the periphery. 

 These masses, at first granular, later became solid and eventually 

 transformed themselves into elongated, thread-like bodies, which 

 were interpreted to be microgametes. This evolution was completed 

 at the end of 18 hours. 



Meanwhile others of the original spores went through a wholly 

 different course of development, which was not at the time considered 

 in detail. It was evident, however, that these elements retained 

 their cytoplasm and eventually transformed themselves into oval 

 cells, with rather dense cytoplasm and a vesicular nucleus containing 

 a large karyosome. These, which were interpreted to be females, 

 also reached the end of their development within 18 hours. 



Finally, appearances suggesting fertilization were noted. 



At the time when this preliminary notice was prepared, my material 

 for the later stages, from 12 to 18 hours, was abundant and the 

 conclusions as published were based on the findings in a number of 

 mice. On the other hand, for the early stages only three mice were 



