THE LIFE CYCLE 573 



with that of the more significant host, for example, with the infection 

 of man in the case of Plasmodium. The psychological effect of this is 

 to deflect interest from the significant biological aspects of such cycles. 

 In order to follow these life cycles in their true biological sequences, 

 we have rearranged them and will now proceed to discuss three of the 

 most widely known ones, viz., those of Eimetia, Plasmodium, and Para- 

 mecium. 



The Life Cycle of Eimeria schiihergi 



The life cycle of Eimevia schubergi, a parasite in the intestinal epi- 

 thelium of Lithohius foyfcatus, is a typical one with an alternation of 

 asexual and sexual reproduction, and of a sexual phase with asexual 

 ones. In this cycle no less than five different structural types of functional 

 individuals appear, each with a distinctive pattern of shape, size, struc- 

 ture, and activity. Four of the five appear but once, but one is subject 

 to numerous repetitions under favorable conditions. 



As rearranged, the biological cycle begins with the zygote formed in 

 the lumen of the intestine of the host by the fusion of a flagellated 

 spermatozoan with a yolk-laden egg, recently emerging from an intestinal 

 epithelial cell of its host. Even before the pronuclei fuse, the fertilized 

 egg forms a fertilization membrane and secretes a cyst wall around its 

 spherical body (Fig. 137, I). Two nuclear divisions bring the organism 

 to the cleavage stage of a four-celled somatella, the sporoblast. There- 

 upon there ensues the first asexual reproduction, when this somatella 

 divides into four unicellular spores. Unlike their spherical parent, these 

 functional individuals are ellipsoidal, and they, too, secrete about their 

 respective bodies a resistant ellipsoidal spore wall. 



There then ensues the second asexual reproduction, when each spore 

 cell divides into two spindle-shaped, naked unicellular sporozoites, re- 

 tained within the spore case and the enveloping cyst wall of the sporo- 

 blast. At about this stage of the cycle, the sporocyst with its eight sporo- 

 zoites in four spores, is discharged from the intestine of its host, and 

 further development ceases until this infective stage is eaten by a Litho- 

 hius. Here the digestive fluids unstopper the cyst, the sporozoites are 

 released from the spores, each escapes singly through the pore and enters 

 an epithelial cell of the intestine of the host where it develops as a 

 trophozoite, changing in pattern from a spindle shape to a spherical one. 

 The organism at this stage is devoid of any special protecting cover and 



