GENERATION, SEX AND ONTOGENY 



219 



uals, then, of course, it is like the others of the slightly com- 

 plex animals in this respect. But as soon as we rise higher in 



A 



FIG. 125. Fertilization of Petromyzon fluviatilis: A, Sperm nucleus in periphery of the 

 egg plasm; B, sperm nucleus in periphery of the egg plasm, and egg nucleus ap- 

 proaching; C-E, fusing of the egg and sperm nuclei, and appearance of the asters; 

 F, cleavage of nucleus. (After Herfort.) 



the scale of animal life, as soon as we study the more complex 

 animals, we find that the egg cells and sperm cells are almost 



