22 PROCEEDINGS OF THE ACADEMY OF [Jan., 



it is to be understood that the figures are, as is usual in such cases, 

 more or less diagrammatic, since it is impossible to reproduce the 

 exact appearances as found under the microscope. 



Mouse 253. — This mouse was killed 6 hours after feeding and its 

 intestine cut into 29 pieces, numbered from 1 to 29. Observations 

 were made on int. 15, 19, 25, 27, and 29. 



The conditions here are, of course, very much like those of mouse 

 250, the cells showing evident males and females along with a number 

 of others which had not evolved far enough to permit of their being 

 determined. Characteristic females are shown in figs. 35, 36, and, 

 Plate III, 37. A male is shown in fig. 38, which was taken from int. 16. 

 This, while differing somewhat from figs. 31 and 32, is nevertheless 

 obviously a male, since the cytoplasm has wholly disappeared and 

 the parasite consists of nothing more than the enlarged nucleus of 

 the original spore. 



Mouse 99. — This mouse was given two infective feeds separated 

 by an interval of 3 hours and killed 3 hours after the second. It 

 therefore represents the conditions at both 3 and 6 hours after 

 inoculation and it is usually possible, in the case of any given parasite, 

 to say to which of the two inoculations it belongs. Thus, fig. 39 is 

 obviously that of a male and no doubt belongs to the 6-hour stage. 



The Later Stages. 



The Male. — The early stages of the evolution of S. muris have, 

 as far as possible, been traced step by step as they occurred in mice 

 killed from 1 to 6 hours after inoculation. At the outset the spores 

 are apparently all alike, although it is possible that the dimorphism 

 may be in evidence even for the spores in the cysts, as appears to be 

 the case for the spores of Sarcocystis colli (Fantham, 1913). But 

 whether this be so or not, by the end of 6 hours at least a considerable 

 proportion of the parasites present are clearly separated into two 

 categories. 



For the later stages it is more convenient to follow these two lines 

 of development separately, instead of considering what is found at 

 the end of increasingly longer periods of time. For after the first 

 eight or ten hours the time factor ceases to be of value, and both 

 9- and 18-hour mice, for instance, may show identical stages of 

 evolution. The development of the male will first be considered. 



As we have already seen, in the first few hours the male parasites 

 lose most if not all of their cytoplasm and become reduced to an 

 element which is obviously only the enlarged nucleus of the original 



