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FERTILIZATION OF THE FROG'S EGG 



Polar bodies 



Animal hemisphere 



Gray crescent 

 Vegetal hemisphere 



Jelly coats (3) 

 itelline membrane 



The egg of the frog 35 minutes after fertUization. Note the second polar body, 

 the slight depression at the animal pole, and the recession of the cortical pig- 

 ment toward the sperm entrance point, forming the gray crescent. The jelly is 

 thicker than the diameter of the egg. 



enters by way of the animal hemisphere. The factors which limit 

 effective sperm entrance to the animal hemisphere may be physical 

 and/or chemical. In any case, the reaction is rapid and the entire 

 egg becomes resistant to excess sperm entrance. In Amphioxus the 

 sperm entrance is characteristically in the vegetal pole, and in other 

 forms there may be no such polar restrictions. 



Sperm contact and penetration of the egg has two rather immedi- 

 ate effects. First, it seems to allow the superficial jelly to swell to its 

 maximum, by imbibition. The jelly on the unfertilized egg will expand, 

 but to a lesser degree. The expansion of the jelly is at quite a uniform 

 rate and can be observed under low power magnification against a 

 dark background. Within 5 minutes there has been about 30 per cent 

 swelling, in 15 minutes about 75 per cent swelling, and thereafter the 

 imbibition is slower. Eventually the thickness of the three jelly layers is 

 several times the diameter of the egg. Second, another effect of sperm 

 penetration is that there is an almost immediate loss of water from the 

 egg so that a space appears between the egg surface and the envelop- 



