REPRODUCTION AXD THE LIFE CYCLE 



181 



is made up of 

 be resolved in 

 classification, 

 and diverse or 

 session of two 

 the other, the 

 latter outside 



heterogeneous and unrelated forms which may in time 

 to more natural o-roups than those of our present-day 

 But notwithstanding- the varieties in form, mode of life, 

 igin of these organisms, all seem to agree in the pos- 

 distinct phases of activity, one, the endogenous cycle, 

 exogenous cycle, the former within the same host, the 

 of any definitive host and either free or temporari y 



Fig. 







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

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

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

 edly (/■'), until many daughter nuclei are formed (G). These become nuclei of ameboid 

 gametes {ID, 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 otfier animal. The life history of Coccidium schu- 

 hergi, 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 v.-irioiis modificMtions. 



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

 history of parasitic protozoa is siinj)lilied to such an extent that no 

 reproductive proces.ses take place (luring the endogenous cycle, the 

 young sporozoites developing directly into trophozoites and these into 



