THE REDUCTION OF THE CHROMOSOMES 



221 



of oogonia (ovogonia). The oocyte, usually while its nucleus is in the 

 prophases of the first maturation division, enlarges greatly ("growth 

 period"), becomes filled with stored food, and develops the general fea- 

 tures characterizing the egg. The oocyte is now called the "ovarian egg," 

 and it actually is an egg in all respects save one of much importance : its 

 nucleus still has the full number of chromosomes. At a comparatively 

 late stage, in many cases even after the spermatozoon has entered the egg 

 at fertilization, the oocyte nucleus (germinal vesicle), having passed 

 through some of the prophasic changes characteristic of the heterotypic 



ANIMAL 



FIG. 



77. Diagram showing the history of the chromosomes in the ordinary 

 life cycles of animals and plants. 



mitosis before and during the growth period, gives rise to a mitotic figure 

 which is often surprisingly small for the volume of the nucleus. The 

 spindle takes up a position perpendicular to the surface of the cell, and at 

 telophase the chromosomes passing to the outer pole are included in the 

 first polar body, a small cell budded off at this point. (See Fig. 106.) A 

 second spindle is rapidly formed about the chromosomes remaining in 

 the egg (called at this stage the secondary oocyte) and the second matura- 

 tion mitosis occurs, one daughter nucleus being included in the second 

 polar body. In the course of these two divisions chromosome reduction 

 is accomplished. The first polar body may divide to form two, thus 

 completing the tetrad of cells corresponding to the tetrad of spermatozoa 

 in the male. Although the polar bodies are normally functionless they are 



