82 FERTILIZATION OF THE FROG'S EGG 



lead the sperm nucleus directly to the egg nucleus, or to the final 

 position of the egg nucleus (which itself moves away from the cortex) . 

 In these cases the sperm path is diverted so that it will meet the egg 

 nucleus. This new direction is designated as the copulation path be- 

 cause it results in the copulation (or fusion) of the two nuclei. By the 

 time the two nuclei meet, the two sperm centrosomes are then well 

 separated and are ready to form the division spindle for the first 

 cleavage. It is probable that the chromosomes from the two sources do 

 not pair off before the first division, which occurs within 2Vi hours 

 after insemination at normal laboratory temperatures. 



Symmetry of the Egg, Zygote, and Future Embryo 



As has been emphasized, the egg polarity and the radial (rotatory) 

 symmetry about its axis are established while it is in the ovary during 

 the process of growth or yolk acquisition. One can draw an imaginary 

 line from the center of the animal pole and through the nucleus which 

 will pass through the geometrical center of the egg, and such a line will 

 pass through the center of the vegetal hemisphere. This line repre- 

 sents the primary axis around which there is radial symmetry. The 

 effective spermatozoon may enter the egg at any point within the 

 animal hemisphere. This may be close to the egg nucleus (toward the 

 center of the animal hemisphere), almost at the equator, or between 

 these extreme positions. It is probably very seldom that penetration 

 occurs just above the second maturation spindle, in the center of the 

 animal hemisphere (i.e., in the egg axis). For this reason, it is safe to 

 assume that the majority of penetration points will be at some other 

 region of the animal hemisphere. This means that a third point is estab- 

 lished (i.e., by sperm penetration), the other two being any two 

 points on the linear axis of the egg. These three points will establish a 

 plane, the penetration path plane, which is of major importance in 

 establishing embryonic planes. The egg therefore loses its rotatory 

 symmetry at the moment of sperm penetration. 



The significance of the sperm entrance point is brought out when it 

 is realized that it immediately establishes the antero-posterior axis or 

 bilateral symmetry of the future embryo. That side of the animal 

 hemisphere which is toward the sperm entrance will be toward the 

 anterior, the opposite will be toward the posterior, of the future 

 embryo. This antero-posterior plane (formerly the sperm penetration 

 path plane) separates the future embryo into right and left halves. 



