110 HEREDITY AND ENVIRONMENT 



cone (Fig. 4), and as soon as the head of the 

 sperm has entered this cone some of the super- 

 ficial 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 re- 

 sult 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 

 evident 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 (Fig. 23 F) . At the same time 

 the chromatin granules and threads in the egg 

 and sperm nuclei run together into a smooth 



