434 PROCEEDINGS OF THE ACADEMY OF [Maj', 



parasites both in 2j- and 3j-hour stages have, at least in a certain 

 proportion of cases, undergone conspicuous changes. These changes 

 consist in a gradual diminution of the quantity of cytoplasm, which 

 seems either largely or completely to disappear, while con- 

 commitantly there is an increase in size and complexity of the 

 nucleus. There is in this way produced a parasite such as is shown 

 in fig. 4, which, so far as both its history and appearance go, is 

 only the nucleus of the original spore. The stage here represented 

 is especially characteristic of the period about six hours after feeding. 

 It may, however, be stated that it is not certain that all of the 

 parasites which invade the cells suffer this loss of the cytoplasm. 



In mice killed nine hours after feeding, this same stage (as shown 

 in fig. 4) may also be found, but it is no longer abundant. This 

 period in the evolution of the parasite, that is, nine hours after 

 feeding, is characterized by a great variety of conditions, of which 

 the majority are difficult to interpret. But by this time it has 

 become evident that the parasites are separating into two categories, 

 which become more and more sharply differentiated as time passes, 

 and which reach their full culmination at the end of 18 hours. The 

 end products of these two lines of evolution are shown in figs. 

 9 and 11, and the interpretation placed upon them is that they are 

 respectively males and females. 



The male elements appear to arise from forms like that shown in 

 fig. 4. These, which apparently consist of only the nucleus of 

 the original spore, show a karyosome, and a nuclear net which here 

 and there supports little aggregates of chromatin. Later stages 

 (fig. 5) show a greater quantity of chromatin, but the karyosome has 

 disappeared. Figure 5 is to be taken merely as representing one of 

 a number of forms which, while differing greatly in detail, agree in 

 that each possesses a nuclear net which supports a quantity of 

 chromatin. In some cases the chromatin occurs in a much coarser 

 form than that shown in fig. 5, whereas in others it is present in 

 ven>^ minute granules distributed throughout the entire extent of a 

 finely meshed net. 



Eventually, however, a stage is reached such as is shown in fig. 

 7. This consists of an oval l)ody with a stringy matrix and a row of 

 granular aggregates arranged around the periphery. These granular 

 aggregates become more and more compact until finally they come 

 to consist of solid, round balls of deeply staining chromatin (fig. 8). 

 These Ijalls, in their turn, elongate and transform themselves into 

 bodies such as are shown in fig. 9, which can scarcely be other 



