REPRODUCTION 



161 



penetrate the eggs. Usually but one sperm cell can enter an egg. After 

 its entrance other spermatozoa are excluded, either by a change in the 

 surface of the egg or by some other mechanism. The spermatozoan 

 nucleus and egg nucleus arrange themselves 

 side by side; and, as the zygote begins to 

 divide in development, the chromosomes 

 of the two nuclei mingle in such a way that 

 their separate sources are as a rule com- 

 pletely obscured. A new cell has arisen 

 from two cells, and out of it comes a new 

 individual derived from two parents. 



Sexual Reproduction in Protozoa. — In 

 some of the protozoa, sexual reproduction 

 involves union between two cells that are 

 alike, which are accordingly known as iso- 

 gametes (Fig. 129). In other unicellular 

 organisms the cells that unite are neces- 

 sarily of different kinds and are then 



known as anisogametes. In Eudorina elegans the difference is one of 

 size; fusion is always between a large cell and a small one (Fig. 130). 

 These might at first seem comparable to the egg and spermatozoon of 



Fig. 129. — Isogamy in Heteromita 

 lens. {After Kent.) 



Fig. 130. — Reproduction in Eudorina elegans Ehrenberg. A, adult colony X 475; B, 

 daughter colony produced by division of one of the cells of such a colony as in yl, X 730; 

 C-E, development of spermatozoa from a mother cell; F, separate spermatozoa. {From 

 West after Goelel.) 



metazoa, but both the large and the small gametes in Eudorina have 

 flagella and are therefore motile. In Volvox and Pleodorina there arc 

 likewise differences in size, and the large cells are nonmotile. Still, the 

 parallel between these large cells and the eggs of metazoa is not complete. 



