HEREDITY 



FIG. 2. Fertilization of the egg of Nereis. 

 A. The sperm has entered the egg and is forming a minute 

 nucleus at o^. The egg-nucleus is breaking up preparatory 

 to the first maturation division. B. The egg-nucleus is 

 undergoing the first maturation division. Notice the con- 

 spicuous rod-like chromosomes separating into two groups. 

 The sperm-nucleus ( ) is now larger and lies deeper in the 

 egg. C. A small polar-cell has been formed above by the 

 first maturation division of the egg. A second division is 

 in progress at the same point. The sperm-nucleus is now 

 deep in the egg and is preceded by a double radiation (am- 

 phiaster). D. Two polar-cells are fully formed. The ma- 

 tured egg-nucleus is now fusing with the sperm-nucleus. 

 An amphiaster indicates that division of the egg will soon 

 take place. (After Wilson.) 



IS 



