22 



THE MAMMALIAN EGG 



remains within the vitellus. The vitelline group of chromosomes 

 now arranges itself as the equatorial plate of the second meiotic 

 spindle, the centromere of each chromosome is split in half and, 



Fig. 14 



Stages of maturation in the rat egg. In (i) to (1), the first polar body is shown with broken 

 outline because it often disappears before ovulation. In (j) to (1), lines in zona pellucida and 

 thickened outline of vitellus indicate occurrence of zona reaction and block to polyspermy, 

 respectively. Shrinkage of vitellus takes place about the time of first-polar-body emission, 

 (f) to (i), and again shortly after sperm entry which is supposed to have happened between 

 (i) and (j). (From Austin, 1959c.) 



at anaphase, the component chromatids are separated to opposite 

 poles of the spindle. Again, one group is expelled, this time within 

 the second polar body, and the other retained in the vitellus. Each 



