18 PROCEEDINGS OF THE ACADEMY OF [Jan., 



the cytoplasm is greatly reduced in quantity and is vacuolated, while 

 in all three the nucleus is enlarged and shows the usual nuclear net. 



Mouse 249. — This animal was killed three hours after feeding. Its 

 intestine was cut into 31 pieces numbered from 1 to 31. Observa- 

 tions were made upon the stomach and upon int. 2, 6, 8, 10, 12, 18, 

 20, 22, 30, and 31. The stomach and the first five pieces of the 

 intestine examined were negative. In int. 18 spores were present 

 in the lumen; in int. 20, 22, 30, and 31 they were found both in the 

 lumen and in the cells. 



It has already been shown (p. 11) that in some of the mice the 

 spores move along the intestine very rapidly, yet it is always to be 

 remembered that the failure to find them in a few selected slides is 

 not proof of their absence from the particular portions of the intestine 

 from which these slides were made. In this mouse it seems unlikely 

 that so much of the intestine was actually negative. 



It is in mouse 249 that the differentiation into males and females 

 first begins to be noticeable. In the case of the males, in which the 

 characteristic changes consist of a great enlargement of the nucleus 

 and a reduction of the cytoplasm, there is no confusion, since these 

 changes are conspicuous and readily detected. 



Thus, Plate II, figs. 18, 19, and 20 are all obviously males, since they all 

 show nuclear enlargement and cytoplasmic reduction. A particularly 

 good earmark of the males is the fact that the nucleus is not sur- 

 rounded by cytoplasm, the parasites consisting (fig. 18) of an enlarged 

 nucleus provided with two tongue- or cap-shaped masses of cyto- 

 plasm. The 'conspicuous black granule, mentioned above as 

 appearing in iron hematoxylin stains, is to be seen in fig. 20, whereas 

 fig. 19 shows a stage wherein the cytoplasm is nearly gone and the 

 nucleus has reached a size equal to that of many of the entire para- 

 sites. 



On the other hand, the differences between the female and the 

 original spore are by no means so striking. The female is relatively 

 shorter and broader, while the nucleus is sometimes larger and, 

 as a rule, shows the chromatin concentrated into a single large 

 karyosome. There is, however, no loss of cytoplasm. Moreover, 

 with an exception to be noted below, the female undergoes no such 

 conspicuous changes in the course of its evolution as does the male. 

 In consequence, in these early stages it is always difficult and some- 

 times impossible to say whether a given parasite is a female or merely 

 one which has been in the cell a short time. 



Hence, it is only with much reserve that fig. 21 , Plate II, may be pro- 



