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 of the spindle. B and C, Later stages showing the 

 separation of the daughter chromoses from one another and their move- 

 ment 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.) 



