THE REPRODUCTION OF ANIMALS 



221 



that follows the nuclear division is so extremely unequal that all of the 

 cytoplasm and yolk remain in one daughter cell, the secondary oocyte, 

 while the other nucleus is extruded onto the outer surface of the oocyte 

 and forms the first polar body. (The polar bodies are named from the fact 

 that they are extruded at the point — nearest the egg nucleus — where the 

 first embryonic divisions will begin, one of the "poles" of the embryo.) 



The second meiotic division soon follows, and the cytoplasmic division 

 of the secondary oocyte is also unequal. Again one of the nuclei is 

 extruded as the second polar body, and the cell that retains all of the 

 cytoplasm and yolk is now a matured egg (ovum), ready for fertilization. 

 The first polar body may also 

 undergo a second meiotic division, 

 in which event the egg will show 

 three polar bodies instead of two. 

 but the polar bodies soon disappear 

 (either by reabsorption or disinte- 

 gration) and have no further 

 significance. 



Oogenesis is evidently adapted to 

 conserve the cytoplasm and food 

 built up in the primary oocyte within 

 a single egg. It is a matter of chance 

 which of the four daughter nuclei is 

 retained in the egg, and which are 

 extruded as polar bodies. 



The matured egg. At the com- 

 pletion of oogenesis the egg has been 

 or is ready to be shed from the ovary. 

 It now consists of a nucleus with 

 the reduced (haploid) number of chromosomes, a comparatively large 

 mass of cytoplasm, and (in most eggs) a supply of stored food material or 

 yolk. The egg is many times larger than the spermatozoon, and is in- 

 variably nonmotile. In several groups of animals the egg is also provided, 

 either before or after fertilization, with such accessory parts as a " white" 

 (additional food material secreted around the egg), a shell, and various 

 supporting, attaching, or protecting structures. 



The maturation of the sperm (spermatogenesis). The primitive male 

 sperm cells (spermatogonia) also multiply in the testis by mitosis, so 

 that enormous numbers of them are provided. Maturation of the sperma- 

 togonia is nearly synonymous with meiosis. As the cell enters the pre- 

 liminary stages of the first meiotic division, it is known as a primary 

 spermatocyte; the first division results in the formation of two similar 

 haploid secondary spermatocytes with equal amounts of cytoplasm. Each 



Fig. 15.5. A small part of a section through a 

 whitefish egg undergoing maturation, show- 

 ing the formation of a polar body. The 

 enormous difference in size between the cell 

 which will become the egg and the sister cell 

 which forms the polar body is entirely due to 

 the unequal distribution of the cytoplasm. 

 The division of chromosomes between the 

 two cells is equal. The cut section does not 

 show all the chromatin of the polar body. 

 (Courtesy General Biological Supply House, 

 Inc.) 



