No. 465.] INTERRELATIONSHIPS OF SPOROZOA. 615 
logeny. That is, the Monocystids have arisen from certain 
sporozoites, which have pushed their way entirely through the 
intestinal epithelium instead of remaining either attached to it or 
within it. Finding in this way a congenial environment, they 
have survived and eventually established a new group of animals. 
It is thus possible to decide which of the two modes of repro- 
duction displayed by these two groups is the more primitive. 
On a priori grounds, heterogamy can be derived from isogamy 
or isogamy from heterogamy. But, presumably, the more 
primitive group will display the more primitive mode of repro- 
duction. Whence the conclusion appears justifiable that the 
lack of sexual differentiation in the Monocystidea is the result 
ofaloss. This conclusion is supported by the fact that, in the 
other Telosporidia, the organisms are male and female. 
The life history of a Coccidian begins in precisely the same 
way as that of a gregarine. A sporozoite, released in the lumen 
of the intestine, seeks and penetrates an epithelium cell. From 
this point, one of two courses may be followed. Easily satisfied, 
the sporozoite may settle down at once within the cell it has en- 
tered, and proceed to grow. Or else it may completely pass 
through the intestinal wall, and eventually come to rest within a 
cell of the liver, kidney, or testis. Ineither case, once established, 
the organism grows until it reaches a certain definitive size. 
Then, by a process termed schizogony, it divides into a number 
of merozoites. These seek fresh cells, which they invade. The 
schizogonous cycle, which may be repeated many times, occa- 
sionally produces the parasites in such numbers that the host is 
kiled. But whether or not, at the end of a certain time, the 
ozoites develop into male and female cells. Each male cell 
produces a number of minute elements, the microgametes. Each 
female cell develops into a single egg, or macrogamete. At 
maturity, the mobile microgametes. seek and fertilize the eggs. 
Immediately after fertilization, the egg lays down a protective 
covering and becomes an oocyst. 
The process differs from that in the Polycystidea in two 
respects. In these, the female trophozoite produces numerous 
eggs and fertilization takes place within the cyst. In the Coc- 
cidia, each female trophozoite metamorphoses into a single egg, 
