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 



