THE MORPHOLOGY OF FERTILIZATION 



6i 



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 



\ 



a 



§ 



b 



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Fig, 7. — Fertilization of the egg of the sea urchin Toxopncustcs 

 (after E. B. Wilson): a, the spermatozoon; h, 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; //, 

 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. 



