3-4 PROCEEDINGS OF THE ACADEMY OF [Jan.. 



Some of the features presented by the evolution of S. muris, 

 however, when considered wholly by themselves, are worthy of 

 attention. One of the most striking of these is the loss of cytoplasm 

 displayed by the microgametocyte. So far as I am aware, no other 

 protozoan displays a mode of development at all like this. Yet it 

 is perhaps not so different from the usual procedure as it may at 

 first seem. In all cases of the evolution of microgametes in Sporozoa 

 the cytoplasm is largely or wholly inert. Thus, in the case used for 

 comparison, that of Coccidium schubergi, the nuclear membrane 

 dissolves and the chromidia collect in the cytoplasm where they 

 eventually produce the microgametes. But there is nothing to 

 show that the cytoplasm takes any active part in this development. 

 It may function merely as a mechanical support for the micro- 

 gametes. Furthermore, at the end of development the microgametes 

 abandon the cytoplasmic mass, which is merely left behind. 



This, indeed, is generally true. Throughout the entire group of 

 the Sporozoa the fully developed microgametocyte consists of a 

 number of microgametes associated with a residuum that represents 

 unused cytoplasm. This residuum is always abandoned, and, so 

 far as it is possible to see, it is always about equal in bulk to that of 

 the original cytoplasmic mass. It may therefore be suggested that 

 in the case of *S. muris, the discarding of the cytoplasm merely takes 

 place before the microgametes are developed instead of afterward. 



The next peculiarity is the fact that there is either only a slight 

 gain of substance, as in the females, or an actual loss, as in the males. 

 This is probably correlated with the speed with which the develop- 

 ment is effected. In general, parasitic Protozoa that live in cells 

 enter these cells as very minute bodies which increase in size for a 

 longer or shorter period, growing at the expense of the cell, and it is 

 only after they have reached a certain size that they divide. That 

 is, the host cells are the dwelling places of these parasites. 



But in the case of »S. muris in the intestinal epithelium of the 

 mouse, the host cell appears to be used merely as a temporary lodging 

 place. The parasites, although they appear usually to destroy the 

 cells they invade, do not seem to feed upon them. No sooner do 

 they come to rest than they proceed forthwith to evolve into the 

 mature sexual stages, which are produced within 18 hours or less. 

 It is apparent that the so-called spore possesses enough energy at 

 the outset to carry on this development, and it seems much more 

 plausible to look upon the granules which the spores contain as 

 reserve food than as the so-called sarcocystin. 



