116 HEREDITY AND ENVIRONMENT 



the point touched a prominence, or reception 

 cone (Fig. 4 A-E), and as soon as the head 

 of the sperm has entered this cone some of the 

 superficial protoplasm of the egg flows to this 

 point and then turns into the interior of the 

 egg in a kind of vortex current. Probably as 

 a result of this current the sperm nucleus and 

 centrosome are carried deeper into the egg and 

 finally are brought near to the egg nucleus 

 (Fig. 23 D and E). In the movements of 

 egg and sperm nuclei toward each other it is 

 probable that they are passively carried about 

 by currents in the cytoplasm; the entrance of 

 the sperm serves as a stimulus to the egg cyto- 

 plasm which moves according to its preestab- 

 lished organization. 



2. Cleavage and Differentiation. — When 

 the sperm nucleus has come close to the egg 

 nucleus the sperm centrosome usually divides 

 into two minute granules, the daughter centro- 

 somes, which move apart forming a spindle 

 with the centrosomes at its poles and with 

 astral radiations running out from these into 

 the cytoplasm (Figs. 3, 4, 23 F) . At the same 

 time the chromatin granules and threads in the 



