THE MORPHOLOGY OF FERTILIZATION 



61 



The sperm nucleus arrives in the egg with its chro- 

 matin in the most condensed condition. The nucleus 

 then begins to enlarge by imbibition of fluid and tends 



FIG. 7. Fertilization of the egg of the sea urchin Toxopnenstes 

 (after E. B. Wilson): a, the spermatozoon; b, c, the sperm head and 

 middle piece immediately after entrance; tail apparently absent; 

 beginning of rotation c; d, rotation half-way completed; origin of sperm 

 aster; e, rotation completed, middle piece separated from sperm centro- 

 some; /, g, approach of the germ nuclei; growth of the sperm aster; h, 

 meeting of the germ nuclei; division of the sperm aster; i, first seg- 

 mentation nucleus in which the sperm and egg components are readily 

 distinguished. 



