GENERAL SKETCH 



139 



The facts just described are now known to be typical of a large 

 number of cases. We may, however, distinguish two types of ferti- 



Fig. 70. — Conjugation of the germ-niiclei and division of the sperm-aster in the sea-urchin 

 Toxopneustes, X 1000. (For later stages see Fig. 37.) 



. /. Union of the nuclei, extension of the aster. D. Flattening of the sperm-nucleus against the 

 egg-nucleus, division of the aster. ' 



lization according as the polar-bodies are formed before or after the 

 entrance of the spermatozoon. In the first case, well illustrated by 

 the sea-urchin (Fig. 69), the germ-nuclei conjugate immediately after 



