EMBRYONIC DEVELOPMENT 



197 



surface and is placed approximately perpendicular to the surface. The 

 tetrads take their place on this spindle, again with their maternal and 

 paternal halves either toward the ends of the spindle or toward its sides 

 (Fig. 167). What kinds of dyads go into the two daughter cells depends 



Fig. 167. — Two possible positions of tetrad on spindle of first division in oogenesis, and 



the kinds of cells resulting from them. 



on which of these two positions the tetrads take. In Fig. 164 the tetrads 

 are assumed to have been turned with their maternal half toward one 

 end of the spindle, the paternal half toward the other, so that the first 

 division was a reduction division. Each dyad formed is thus either 

 wholly maternal or wholly paternal, although of the 

 dyads in a given cell some may be paternal, some 

 maternal. 



The two cells are of very unequal size. One 

 contains nearly all the protoplasm of the primary 

 oocyte, the other very little indeed. The disparity 

 between them is much greater than Fig. 164 indicates; 

 the correct sizes for one animal are shown in Fig. 168. 

 The larger cell is named the seco7idary oocyte. The 

 smaller cell is never functional and is called the 

 first polar body or first polocyte; it eventually 

 degenerates. 



In most animals only the secondary oocyte undergoes further division. 

 In some species the first polar body also divides, and, to complete the 

 comparison with the male, this occasional division is represented in Fig. 

 164, but the resulting two polar bodies are not functional. 



Fig. 168.— Star- 

 fish egg with 

 polar body above. 

 (Courtesy of General 

 Biological Supply 

 House.) 



