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FERTILIZATION OF THE FROG S EGG 



involves the loss of pigment from the surface of the egg, there appears 

 a compensatory marginal region between the animal and the vegetal 

 hemispheres which is neither black nor white, but is gray. This inter- 

 mediate shading is due to the partial loss of surface pigment and the 

 consequent greater exposure of the underlying yolk, so that it is known 

 as the gray crescent. It establishes the gray crescent plane, and is 

 known as the plane of embryonic symmetry. The gray area has a 

 crescentic shape, due to the spherical surface involved. The surface 

 pigment is largely in a separate non-living coat, but one which is 

 necessary for the maintenance of cell integrity and is actively involved 

 in the subsequent cleavage process. But this sheet of pigment is pulled 

 slightly in the direction of the penetrating sperm which carries part of 

 the pigment into the egg. The opposite side of the sheet of pigment 

 is therefore pulled away from the underlying yolk, leaving it partially 

 exposed. The gray crescent is entirely a surface phenomenon and has 

 nothing whatever to do with syngamy (the fusion of gametes). Even 

 if the sperm aster formation is experimentally inhibited, the gray 

 crescent will form, it being a response to sperm penetration. The region 

 of the gray crescent will become the posterior side, and the opposite 

 (region of sperm entrance) will become the anterior side of the future 

 embryo. If we now take any two points on the egg axis, and the center 

 of the gray crescent as a third point, this plane represents the median 

 sagittal plane of the future embryo. It must be pointed out that the gray 

 crescent is not always readily apparent, and yet such eggs will develop 

 normally. 



It is presumed that the closer to the equator that the sperm makes 



Formation ol the gray crescent. (A) Egg of Rana pipiens at the moment of 

 insemination. (B) Same egg 20 minutes later showing the iormed gray crescent. 



