BEHAVIOR OF THE GAMETES 249 



a gray appearance. This area is crescentic in shape and is known as the gray 

 crescent (fig. 1 lOK). 



The formation of the gray crescent occurs in the cytoplasmic area just 

 above the margin where the yellow-white vegetal pole material merges with 

 the darkly pigmented animal pole. The gray crescent is continuous with the 

 lighter vegetal pole material and is seen most clearly during the first cleavage 

 of the egg. The plane which bisects the gray crescent into two equal halves 

 represents the future median plane of the embryo. 



In the frog, Rana jusca, Ancel and Vintemberger ('33) have shown that 

 extensive movements of egg-surface materials accompanies the formation of 

 the gray crescent. Sperm contact with the egg's surface thus appears to set 

 in motion ooplasmic substances which fix the final symmetry of the egg and 

 the future embryo. 



4) Fertilization of the Teleost Fish Egg. When the egg of the teleost fish 

 is spawned, the yolk lies near the center of the egg, and its yolk-free cyto- 

 plasm forms a peripheral layer. Around the egg the yolk-free cytoplasm is 

 somewhat more abundant in the region where the egg nuclear material is situ- 

 ated. This concentration of the peripheral cytoplasm at the nuclear pole is 

 more evident in the eggs of some species than in others. The area of nuclear 

 residence is situated near the micropyle in many teleost eggs, but not in all. 

 For example, the concentration of cytoplasm with the contained nuclear ma- 

 terial is located in Bathygobius soporator at the opposite end to the micropyle 

 (Tavolga, '50). (See fig. 123A.) 



The sperm enters the egg through the micropyle (figs. 122, 123, 134A), 

 and the actual processes of fertilization are initiated when the sperm makes 

 contact with the peripheral ooplasm near the point where the egg nuclear 

 material is located. This normally occurs in about a minute or less after the 

 egg reaches the water. Within a few minutes the second polar body is given 

 off. Meanwhile, the peripheral cytoplasm flows toward the area where the 

 sperm has made contact, and a protoplasmic cap forms at this pole (figs. 

 122C; 123B-D). The remainder of the egg, with the exception of a thin 

 layer of surface protoplasm, contains the deutoplasmic or yolk material. The 

 egg is converted in this manner from a more or less centrolecithal egg into a 

 strongly telolecithal egg. (Compare with Stye la and Amphio.xus.) 



While these events are progressing, the egg as a whole contracts slightly, 

 and a fluid is given off into the forming perivitelline space between the egg's 

 surface and the vitelline membrane (fig. 122B, C). (However, a space be- 

 tween the egg membrane and the egg is evident to some extent in certain 

 teleost eggs before the sperm enters the egg (fig. 123A).) The egg is now 

 free to rotate vvithin the perivitelline space, being cushioned and bathed by 

 the perivitelline fluid. 



The expansion of the vitelline membrane of the egg in certain teleosts is 

 both dramatic and prophetic of the future shape of the embryo (fig. 123B-H). 



