FERTILIZATION 605 



sary to look further for the effecting stimulus in the Coccidiomorpha. 



The phenomenon is more easily explained in these forms by the inter- 

 pretation of Maupas, which has since been developed especially by Cal- 

 kins, that internal factors play the determining role. This would mean 

 that when the protoplasm had reached a certain degree of maturity in its 

 cycle of development, the sexual phase would be initiated, even though 

 the external conditions remained unchanged. 



Whatever the cause, gametocytes appear and are differentiated into 

 male and female gametocytes. The macrogametocytes are said to eliminate 

 their karyosomes to accomplish reduction. By this process they are trans- 

 formed into large, yolk-filled, egg-like macrogametes. The nuclei of the 

 microgametocytes are said to give off chromidia and then degenerate. 

 The chromidia condense into a number of clusters to form the nuclei of 

 small, sperm-like, flagellated microgametes. A macrogamete is found 

 and fertilized by a microgamete, and the resulting zygote forms an 

 oocyst. The synkaryon divides twice to produce four sporoblasts, each 

 of which now develops two sporozoites. 



A more thorough cytological study of the cycle may eventually reveal 

 chromosome reduction taking place in the two divisions of the syn- 

 karyon, in which case meiosis would be zygotic; or in nuclear divisions 

 prior to gamete formation, in which case it would be gametic. Karyo- 

 some extrusion and the formation of gamete nuclei from chromidia 

 cannot be accepted today as conclusive evidence of meiotic reduction. 

 If, indeed, no chromosomes are formed in Eimeria schubevgi, then we 

 shall be forced to modify our concept of meiosis. Here again we find 

 urgent need for the application of improved techniques in cytological 

 studies of a fundamental nature. 



The sexual processes in the Adeleidea differ from those of the other 

 Coccidia in several interesting respects. In Adelina dimidiata, according 

 to Schellack ( 1913) , two gametocytes of different sizes unite in a pseudo- 

 conjugation process similar to that of the gregarines. The nucleus of the 

 microgametocyte divides twice, and one of the nuclei enters and fertilizes 

 the macrogamete. In this species only one macrogamete is formed and it 

 is fertilized by one pronucleus of the microgamete in a way similar to 

 anisogamous conjugation in the Vorticellidae. The peculiar behavior 

 of the ciliate Metopus sigmoides (see p. 622 below) in conjugation 

 also resembles the sexual union of A. dimidiata. 



