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TEXT-BOOK OF EMBRYOLOGY. 



burrows through the gelatinous capsule and thin vitelline membrane of an 

 egg and enters the cytoplasm usually about 40 degrees from the center of 

 the animal pole. There seems to be some determining factor in the entrance 

 of the sperm at or near that particular parallel, but the point of entrance may 

 lie in any meridian of the egg. The first sperm that enters the cytoplasm 

 seems to set up changes, probably of a physico-chemical nature, which bar 

 admittance to other sperms. The sperm head and the body containing the 

 centrosome move through the cytoplasm for some distance toward the 

 center of the egg, then rotate so that the body is in advance of the head and 

 change their course in the direction of the egg nucleus. The trail of the 

 sperm is marked by an extra amount of pigment, indicating probably some 



B 



FIG. 27. A frog's egg before and after fertilization, showing the formation of the gray crescent. 

 A, Unfertilized egg seen from the side; B, unfertilized egg seen from the vegetal pole. C, 

 fertilized egg seen from the side; D, from the vegetal pole, c, Gray crescent; w, non- 

 pigmented vegetal pole. Kellicott. 



increase in cytoplasmic activity. The course of the sperm toward the 

 center of the egg is the penetration path, the course toward the egg nucleus, 

 the copulation path. 



The sperm nucleus, as soon as it enters the egg, appears to stimulate 

 the cytoplasm to activities leading to a rearrangement of the egg substances 

 and thus to a reorganization. Beginning at the point where the sperm 

 enters, the cytoplasm streams toward the animal pole and the yolk toward 

 the vegetal pole, a sharper polar differentiation thus resulting. On the 

 supposition that this influence of the sperm spreads like a wave from the 

 point of entrance, it follows that the original rotatory symmetry of the egg is 

 disturbed and a new symmetry established which is a bilateral one, with the 

 plane containing the penetration path as the median plane. In other words 



