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Heredity and Environment 



FIG. 4. FERTILIZATION OF THE EGGS OF STAR-FISH AND SEA-URCHIN. 

 A-C, Successive stages in the entrance of a spermatozoon into the egg of 

 the star-fish, Asterias glacialis. Only one sperm has penetrated the jelly 

 layer (//) which surrounds the egg and the peripheral protoplasm (pp) of 

 the egg protrudes as an entrance cone (ec) to meet it. (After Fol.) 

 D, Mature spermatozoon of the sea-urchin, Toxofrneustes, showing head 

 (A) ; middle-piece (m) ; and tail (f). E-H, Successive stages in the pene- 

 tration of the sperm nucleus ( $ N) and centrosome (#C) into the egg of 

 Toxopneustes. I-L, Stages in the approach of the sperm nucleus ($N) 

 to the egg nucleus ($AO, and in the division of the sperm centrosome 

 ( $ C) and the formation of the first cleavage spindle. (D-L after 

 Wilson.) 



