220 



THE CONTINUITY OF THE RACE 



first polar 



body 



ivision of 

 first polar body 



O JL. second polar 

 body 



the fertilized egg 

 or zygote 



Fig. 15.4. Maturation of the sperm and egg, and fertilization. The meiotic divisions of the 

 nuclei, which are much more completely shown in Fig. 15.3, are indicated by symbols for 

 the same three pairs of chromosomes in the primary oocyte and spermatocyte. The chro- 

 matids of each pair are given a special shape (square, round, triangular), with sister chro- 

 matids of the same color and homologous pairs contrasted in black and white. No crossing 

 over is shown, and the symbols should be thought of as cross sections of chromosomes made 

 very near their centromeres, since both first divisions are shown as separating homologous 

 chromatids and the second divisions as separating sister chromatids. The diagrams of 

 spermatogenesis and of the polar bodies are proportionately much too large as compared 

 with the size of the egg. 



process, which begins and is usually completed before meiosis, is followed 

 by a migration of the egg nucleus (and much of the cytoplasm in eggs 

 with large amounts of yolk) to a point near the cell membrane. 



The enlarged egg is now known as a "primary oocyte and is ready for 

 the first meiotic division. When this takes place, the cytoplasmic division 



