THE EARLY DEVELOPMENT OF THE FROG 



85 



downward so that the animal pole is largely freed from yolk, the 

 rial pole composed more largely of it, and the polar dif- 



V( 



ferentiation thus more marked than heretofore. The pigment 

 granules, whose specific gravity is really intermediate between 

 that of the yolk and of the protoplasm, show little disturbance 

 and redistribution except in one certain region. For some 

 reason which is not clear, the pigment granules located in a defi- 

 nite area at the lower margin of the pigmented pole, on the side 



FIG. 29. Frog's egg before and after fertilization, showing the formation of 

 the gray crescent. A. Unfertilized egg, from side. B. Unfertilized egg, from 

 vegetal pole. C. Fertilized egg before first cleavage, from side. D. Same from 

 vegetal pole, c. Gray crescent; p, pigmented animal pole; w, unpigmented 

 vegetal pole. 



opposite that where the sperm has entered, and therefore in 

 the region presumably the last to be affected by the sperm, are 

 carried away from their original position leaving this region 

 lighter in color. This area is crescentic in outline, the crescent 

 extending one-half to two-thirds around the egg; it is known 

 as the gray crescent (Fig. 29). 



The rearrangement of substance which involves the forma- 

 tion of the gray crescent, is such that the. center of the specif- 

 ically lighter substance of the egg is not located precisely in the 

 egg axis, toward the animal pole, but is displaced toward that 



