REPRODUCTION AND THE LIFE CYCLE 



179 



Sexual reproduction is bound up with spore formation, whereby germs 

 of the parasite are prepared to withstand various unsuitable conditions 

 of the external environment, such reproduction being termed sporo- 

 yony. Asexual reproduction, on the other hand, taking place w r ithin 

 the host, is a means of spreading the infection among different cells 



FIG. 74 



Life cycle of Coccidium schubergi. (After Schaudinn.) Sporozoites penetrate epithelial 

 cells, and grow into adult intracellular parasites (a). When mature, the nucleus divides re- 

 peatedly (6), and each of its subdivisions becomes the nucleus of a merozoite (c). These enter 

 new epithelial cells, and the cycle is repeated many times. After five or six days of incuba- 

 tion, the merozoites develop into sexually differentiated gametes; some are large and well 

 stored with yolk material (d, e, f); others have nuclei which fragment into many smaller par- 

 ticles ("Chromidien"), each granule becoming the nucleus of a microgamete or male cell (d, 

 h, i, j). The macrogamete is fertilized by one microgamete (g), and the copula immediately 

 secretes a fertilization membrane which hardens into a cyst. The cleavage nucleus divides 

 twice, and each of the four daughter nuclei forms a sporoblast (A:) in which two sporozoites are 

 produced (0- 



and organs in the same host, or a means of auto-infection. This means 

 of asexual increase is termed schizogony, although, as a rule, the term 

 is restricted to multiple increase or asexual "spore" formation. 



Similar alternations of sexual and asexual methods of reproduction 

 are invariably present in free forms of protozoa, but asexual increase 



