14 



Heredity and Environment 



Fig. 5. First Cleavage of the Egg of the Sea-urchin, Echinus micro* 

 tubcrculatus. A, Nuclear division figure, or mitotic spindle with a centro- 

 some at each pole and with the chromosomes from the egg and sperm 

 nuclei at the equator. B and C, Later stages showing the separation of 

 the daughter chromosomes from one another and their movement toward 

 the two poles of the spindle. D and E, Still later stages showing the 

 swelling of the chromosomes and their fusion to form nuclear vesicles. 

 F, Complete division of egg into two cells, each containing one daughter 

 nucleus and centrosome. (After Boveri.) 



