FERTILIZATION BY EXOGAMY 



153 



1. Isogamy. Not only may isogamous conjugation occur bet ween 

 full-sized and reduced-sized individuals, but among the former there 

 may be a further difference in that the conjugating cells do not fuse 

 to form a zygote, but separate after a few hours (copulation). This 

 process is particularly characteristic of the infusoria and is not met 

 with elsewhere. 



FIG. 66 



Copromonas subtilis. (After Dobell.) .1, normal adult cell before division, from life; B. 

 cells in conjugation, one flagellum being withdrawn; C, fusion, first stage in "nuclear reduc- 

 tion;" D, heteropolar division of nuclei for second "reducing division;" E, fusion of nuclei 

 and formation of cyst; F, fertilized cell in permanent cyst. 



() The Union of Full-sized Cells. With the exception of the lower 

 flagellates, there are few instances of conjugation among full-sized 

 individuals. It has been described by Schaudinn ('96) in the case of 

 the heliozoon Actinophrys sol (Fig. 65), where the two cells fuse after 

 a preliminary process of maturation. Here there is little change in the 

 normal aspect of the two conjugating cells beyond the withdrawal of 

 the pseudopodia and secretion of a protective cyst. So, too, among 

 some of the flagellates there is little difference in the gametes from the 

 normal. In Bodo saltans (Dallinger) they are all alike, while in Copro- 

 monas subtil is, according to Dobell ('08), one of the two cells is absorbed 

 in the other, and its flagellum is lost, while the flagellum of the other 



