FERTILIZATION 



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FIG. 88. Fertilization in the sea-urchin, Toxopneustes. From Wilson, "Cell." 

 A-F, X 1067 -,G, X 533 ;#, /, X 667. A. Sperm-head before entrance; n, nucleus; 

 m, middle-piece and part of the flagellum. B,C. Immediately after entrance, 

 showing entrance-cone. D. Rotation of the sperm-head, formation of the 

 sperm-aster about the middle-piece. E. Casting off of middle-piece; centro- 

 some at focus of rays. F,G. Approach of the pronuclei; growth of the aster. 

 H. Union of pronuclei. /. Flattening of the sperm pronucleus against the egg 

 pronucleus; division of the aster. 



