SYMMETRY OF THE EGG, ZYGOTE, AND FUTURE EMBRYO 83 



Vitelline Polar bodies 



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Surface changes of the frog's egg at the time of fertilization. (A) Sperm 

 contact, entrance generally in the animal hemisphere. (B) Second polar body 

 emerges, the sperm penetrates the cortex, and the surface pigment recedes 

 toward the sperm entrance point forming the gray crescent. Lateral view. 

 (C) Dorsal view showing the sperm penetration path, centrally located polar 

 bodies and egg nucleus, and (arrows) pigment recession to form the gray cres- 

 cent. (D) Rotation of the sperm head and middle piece 180° as it veers from 

 the penetration path to the copulation path, in line with the egg nucleus. (E) 

 First division spindle forming at right angles to the copulation path. Since the 

 copulation and penetration paths are not in the same plane, the cleavage furrow 

 does not cut through the middle of the gray crescent. (F) When the penetration 

 and copulation paths are continuous and in essentially a straight line, the first 

 cleavage furrow will bisect the gray crescent. 



Immediately upon invasion by the spermatozoon tiie egg substance 

 becomes more labile and there is a streaming of the protoplasm toward 

 the animal pole and of the yolk (deutoplasm) toward the vegetal pole. 

 Polarity is therefore accentuated. The movement of the egg contents 

 is reflected in the very violent activity of the microscopic pigment 

 granules on the surface. These granules appear motionless in the un- 

 fertilized or the dead egg. These facts have been established by cyto- 

 logical and experimental investigations, and observed in high mag- 

 nification motion pictures. However, since the sperm penetration 



