THE BRIDGE OF LIFE 495 



Oogenesis 



Oogenesis means egg production. A cell of the ovary which is 

 preparing to undergo meiosis is known as a primary oocyte. The first 

 division of meiosis in this cell differs from that of the primary sperma- 

 tocyte in that the spindle figure is not across the center of the cell, 

 but near one edge. When the two groups of chromosomes have 

 separated, one group of chromosomes is included in a small cell which 

 is formed at the edge of the larger cell. The small cell is known as the 



Courtesy General Biological Supply House 



Fig. 31.4. Unequal cell division in oogenesis of the whitefish. The polar body is 

 being pinched off from the much larger oocyte, yet it receives the same number of 



chromosomes. 



first polar body, and the large cell becomes the secondary oocyte. The 

 second division of meiosis is also unequal, and a large ootid and a second 

 polar body are formed. The first polar body usually divides again also 

 so we end up with three small polar bodies and one ootid. The ootid 

 then forms the egg, and the polar bodies disintegrate. 



The advantage of this unusual type of division becomes apparent 

 when we consider the functions of the egg. The egg must store food 

 for the developing embryo and, therefore, must be larger than the 

 sperms which need only enough food for energy to swim to the egg. 

 By these unequal divisions there is one final cell which is much larger 



