18, 4 Haughwout: Human Coccidiosis 475 
in Isospora there are two tetrazoic spores, there being eight 
sporozoites formed in each case. In the intestine the vermi- 
form sporozoites emerge from their containing cysts and invade 
the epithelial cells of the mucosa of the small intestine. In 
some instances,the epithelium of the bile ducts is invaded, but 
this seems not to be the case with species of Isospora, which 
seem to restrict their activities to the intestine. They are 
obligatory epithelial-cell parasites. Within the host cell the 
sporozoites develop into trophozoites, which grow at the expense 
of the protoplasm of the host cell. When the food supply has 
become exhausted, and the trophozoite has attained full growth, 
the nucleus undergoes multiple division (schizogony) and with 
plastogamy a number of daughter cells, or merozoites, are 
formed.® These escape from the host cell and enter the bowel 
lumen to seek new host cells. Those that are successful, in 
their turn develop into trophozoites which also undergo schi- 
zogony. This asexual cycle is repeated a variable number of 
times, depending upon the vitality of the race,’ and finally 
changes take place leading to a sexual process (sporogony), 
which is initiated by a process of fertilization involving the 
union of sexually differentiated gametes. This is followed by 
the encystation of the zygote, which then passes out of the 
feeces and completes its development in the outer world. It 
so comes to pass that in the course of events the entire race, 
in time, undergoes sporogony and the host becomes purged of 
his infection. In this respect, the infection may be said to be 
self-limited, and if the host can withstand the initial on- 
slaught, his chances for recovery are good. No immunity 
is conferred, however. In a large proportion of cases the host 
seems to undergo no marked discomfort, while in others, as in 
the case of the karyozoic parasite Cyclospora caryolytica of the 
ground mole, the symptoms may be very acute and rapidly 
progress to a fatal termination. In other cases, the infection 
may drag along for a long period of time, the host constantly 
discharging cysts. In such cases, however, it is necessary to 
rule out reinfection with a new crop of parasites. 
* Porter says fifteen to twenty merozoites are produced from one schi- 
zont in the Isospora of man. I presume she bases this statement on the 
infection experiments of Fantham with kittens, the report of which I regret 
to say I have not seen. 
*Some authors maintain that “virulent” infections leading to extensive 
destruction of tissue bring about a condition which stimulates the develop- 
ment of sporogony and thereby relieves the situation. 
