604 FERTILIZATION 



zygotic reduction, but actual separation of chromosomes in the reduction 

 division must be observed before the case is considered to be proved. 

 M. arndti may have zygotic reduction, as the evidence indicates, but Na- 

 ville's interpretation may apply to this case, so that gametic reduction 

 remains a possibility. 



Gamete formation by endogenous budding was found by Leger 

 (1907) to occur in Ophryocystis mesnili (Fig. 144). In this form two 

 gamonts adhere in pseudoconjugation, and the nucleus to each divides 

 twice, one product of each division being destined to degenerate. These 

 are presumably reduction divisions, although cytological evidence for 

 this is lacking. One product of the two divisions becomes the pro- 

 nucleus of the single large gamete which is formed inside the gamont 

 as a loose internal bud. The walls between the gamonts break down 

 and the two isogametes fuse. The zygote thus formed develops a spore 

 wall, and eight sporozoites are produced by metagamic divisions. 



While isogamy is most frequently observed in the gregarines, as for 

 instance in the species already named and in Diplocystis schneideri 

 (Jameson, 1920), Gregarina cuneata (Milojevic, 1925), Actmocephalus 

 parvus (Weschenf elder, 1938), and others, several species show various 

 degrees of anisogamy. Species other than Urospora lagidis, already men- 

 tioned, are Echinomera hispid a (Schellack, 1907), Stylocephalus longi- 

 collis (Leger, 1903), and N'lni gracilis (Leger and Duboscq, 1909), 

 the last of which shows a marked degree of differentiation between the 

 microgametes and macrogametes. This differentiation approaches that 

 usually seen in the Coccidiomorpha. 



An extreme differentiation of gametes (oogamy) is seen in Eimeria 

 as well as in other Coccidia and in many Haemosporidia. The type of 

 syngamy observed by Schaudinn (1900) in Eimeria schuhergi will serve 

 to illustrate this group. Here, as in other cases in which accurate chromo- 

 some determinations have not been made, it is assumed that reduction 

 is gametic. The sexual phase starts with some of the merozoites develop- 

 ing into gametocytes, instead of repeating their asexual cycle. It is diffi- 

 cult to explain on a purely environmental basis why some merozoites 

 repeat the asexual cycle while others, obviously in the same environ- 

 ment (intestinal epithelium) develop into gametocytes. If external 

 factors play the chief role in determining whether a protozoan will con- 

 tinue asexual multiplication or enter a sexual phase, then it will be neces- 



