FERTILIZATION 601 



SPOROZOA 



Among the Sporozoa fertilization is almost universally present. As 

 would be expected in such a heterogeneous group, all kinds of fertili- 

 zation processes are known. Isogamy, heterogamy of all degrees of dif- 

 ferentiation, pseudoconjugation, gametic meiosis, zygotic meiosis, and 

 many other variations of the fertilization process have been described. 

 Naturally only a few typical examples, illustrating the chief types of 

 these phenomena, can be mentioned here. 



Monocystis, the gregarine parasite in the seminal vesicles of the earth- 



rtv-f'-.= 





jel 

 "c^) 



Figure 143. Monocystis ro strata. A and B, metaphase and anaphase of early progamous 

 divisions of pseudoconjugant, eight chromosomes splitting, eight going to each pole; C and 

 D metaphase and anaphase of last progamous (reduction) division, paired chromosomes 

 disjoining, four going to each haploid pole. (After Mulsow, 1911.) 



worm, illustrates typical pseudoconjugation, gametic meiosis, and isog- 

 amous fertilization. Two adult gregarines come together and are en- 

 closed in a common cyst, but do not fuse. This intimate association with- 

 out protoplasmic union is pseudoconjugation, and the members of this 

 chaste betrothal are now gametocytes. The nucleus of each gametocyte 

 divides again and again to form a large number of small nuclei, which 

 migrate to the periphery and eventually become the gamete nuclei. Ac- 

 cording to Mulsow (1911), reduction occurs in the last of these divi- 

 sions before formation of the pronucleus, in Monocysth rostrata. The 

 earlier mitoses (Fig. 143) show eight thread-like chromosomes which 

 split longitudinally, eight halves going to each daughter nucleus. In the 

 last division the eight chromosomes associate in four pairs. In the ana- 

 phase that follows, members of the pairs separate and pass to different 

 poles, thus reducing the number of chromosomes from eight to four. 

 The surface of the gametocyte produces many small buds, each contain- 



