THE MECHANISM OF SEX-DETERMINATION 37 



But before this time the egg-cells divide, like all the 

 other cells of the body. In this way a large number 

 of eggs is produced. After a time they cease to divide 

 and begin to grow larger, laying up yolk and other 

 materials. At this time, the chromosomes unite in 

 pairs, so that their number seems to be reduced to half 

 the original number. At the final stage in the matura- 

 tion of the egg, two peculiar divisions take place that 

 involve the formation of two minute cells given off at 

 one pole — the polar bodies. In some eggs, as in the 

 sea urchin, the polar bodies are given off while the egg 

 is still in the ovary and before fertilization ; in other 

 eggs, as in the frog, one polar body is given off before 

 fertilization, the other after the sperm has entered ; 

 and in other eggs, as in nereis (Fig. 19), both polar 

 bodies are given off after fertilization. 



The formation of the polar bodies is a true cell- 

 division, but one that is unique in two respects. 

 First, one of the cells is extremely small, as seen in 

 Fig. 19. The smallness is due to the minute amount 

 of protoplasm that it contains. Second, the number of 

 chromosomes at each division is the half or ^'haploid " 

 number. There is much evidence to show that at one 

 or at the other of these two divisions the two chromo- 

 somes that had earlier united are separated, and in this 

 respect this division differs from all other cell-divisions. 

 In consequence, the egg nucleus, that re-forms after the 

 second polar body has be^n produced, contains only 

 half the actual number of chromosomes characteristic 

 of all the other cells of the female. 



In the formation of the spermatozoa a process takes 

 place almost identical with the process just described 



