404 BIOLOGY OF THE PROTOZOA 



zoite grows into an intracellular parasite termed the trophozoite, 

 which fills the greater part of the cell and forces the cell nucleus to 

 one side where it degenerates. The trophozoite grows at the 

 expense of the host cell, and when fully grown its nucleus divides 

 a number of times and the cell body divides into a number of 

 daughter cells. This process is termed schizogony or asexual repro- 

 duction and the products are called merozoites. These are liberated 

 into the lumen of the intestine, where they behave exactly like the 

 sporozoites entering epithelial cells which they, also, destroy and 

 grow into adult merozoite-forming trophozoites. The process is 

 repeated a number of times and in this way, by multiple progression, 

 great areas of normal cells are infected and destroyed. Ultimately 

 the merozoites give rise to trophozoites which have a different fate. 

 Some of them grow to full size and as macrogametocytes store up 

 reserve nutriment and become differentiated as macrogametes. 

 Others grow in like manner but instead of storing nutritive sub- 

 stances become free of granules and appear hyaline. The nucleus 

 ultimately begins to divide and its divisions are repeated until many 

 hundreds are present and distributed around the periphery of the 

 cell. These become the nuclei of microgametes which are delicate 

 hair-like cells, each with 2 flagella, distributed over the surface of 

 the mother cell or microgametocyte (Fig. 173, j). 



Immediately after fertilization by union of a macrogamete and 

 a microgamete, a fertilization membrane is formed around the 

 zygote. This membrane becomes hardened into the oocyst or outer 

 protective covering. The zygote is then ready to undergo meta- 

 gamic divisions, first into 2 and then into 4 cells. Each of these is 

 a sporoblast which secretes a protective membrane about itself — 

 the sporocyst— and then divides into 2 daughter cells, each of which 

 is a sporozoite (Fig. 173, /). Each zygote thus gives rise to 4 sporo- 

 blasts with their sporocysts, and to 8 sporozoites, 2 to each sporoblast. 



While details vary widely such a general outline of the life history 

 may be applied to all types of Coccidiomorpha. Variations occur 

 in all phases, particularly in the sporogony cycle, where we find 

 wide differences in the number of sporoblasts formed from the 

 zygote, and in the number of sporozoites formed from each sporo- 

 blast (see Key, p. 557). In the majority of cases the full life history 

 is completed within one host but in a few cases among Coccidia 

 and in all Ilemosporidia two different hosts are necessary, in one, 

 and presumably the original host, only sporogony or sexual phases 

 occur, in the other the usual asexual development and multiplica- 

 tions of the trophozoites. (See p. 406 for adaptations in Hemo- 

 sporidia) . 



In general the effects produced by Coccidia are determined by 

 the extent of multiplicative reproduction and the area of devastated 

 cells. The centipede is little affected and so are the great majority 



