REPRODUCTION AND THE LIFE CYCLE 



181 



is made up of heterogeneous and unrelated forms which may in time 

 be resolved into more natural groups than those of our present-day 

 classification. But notwithstanding the varieties in form, mode of life, 

 and diverse origin of these organisms, all seem to agree in the pos- 

 session of two distinct phases of activity, one, the eiulogenou.?- cycle, 

 the other, the exogenous cycle, the former within the same host, the 

 latter outside of any definitive host and either free or temporarily 



Fig. 7.5 



Life cycle of Lanke.steria (Monocystic ascidiae, Siedlecki). The young sporozoites enter epi- 

 thelial cells (A, B, C), and grow into adult gregarines, which leave the ceils {D) and live as 

 "sporonts" in the cavity of the intestine. Two sporonts unite {E), their nuclei divide repeat- 

 edly {F) , until many daughter nuclei are formed (6=). These become nuclei of ameboid 

 gametes {H) , which move about inside of the cyst and soon conjugate two by two (/), the 

 nuclei fusing to form cleavage nuclei of the sporoblasts {J). The cleavage nuclei then 

 divide thrice to form eight daughter nuclei {K, L, M, N) , which ultimately become nuclei 

 of the sporozoites (O). The sporoblasts, meanwhile, secrete firm cysts within which the 

 sporozoites are protected. 



parasitic in some other animal. The life history of Coccidmm schit- 

 hergl, as outlined above, is neither the simplest nor the most compli- 

 cated of these histories, and may well serve as a starting point for a 

 description of the various modifications. 



1. Variations in the Endogenous Cycle. — In some cases the life 

 history of parasitic protozoa is simplified to such an extent that no 

 reproductive processes take place during the endogenous cycle, the 

 young sporozoites developing directly into trophozoites and these into 



