146 



GENERAL PRINCIPLES OF ZOOLOGY. 



1. Maturation. 



The egg with the large vesicular nucleus (germinal vesicle) 

 cannot yet be fertilized; to render it capable of fertilization it 

 must undergo a series of changes the process of maturation, 

 which consists in the replacement of the germinal vesicle by a 

 much smaller egg-nucleus, and simultaneously the formation at 

 one pple of the egg of the ' directive corpuscles' or ' polar bodies/ 



Formation of the Polar Bodies. The germinal vesicle initiates 

 the changes, its walls disappearing, its contents in part mingling 

 with the cytoplasm of the egg, in part being employed for the 

 formation of a nuclear spindle (directive spindle). The latter 

 places itself with its axis in a radius of the egg so that one pole 

 is turned towards the centre, the other being in the superficial 

 layer of the egg (fig. 92, a). Now begins a regular cell-division,. 





FIG. 92. Successive stages in the formation of the polar bodies of Ast erias glacialis. 

 sp, directive spindle; rfc 1 , first polar body; r/c 2 , second polar body; e/c, egg-nucleuss 

 in process of formation. 



but the products of the division are of very unequal size; the larger 

 part is the egg, the smaller quite insignificant part is the polar 

 body (fig. 92, b, c). The latter projects above the surface carrying 

 with it one half of the spindle, and when the globule is cut off half 

 of the spindle is included in it. 



Tlie Second Polar Body. The part of the directive spindle 

 remaining in the egg immediately forms a new spindle; the cell- 

 budding is repeated and leads to the formation of the second polar 

 body. As a result two small cells (fig. 92, d, e, f) lie at one pole 

 of the egg, in many cases even three, since during the formation 

 of the second polar body the first may have again divided. ^ The 

 part of the directive spindle still remaining after the second divi- 

 sion becomes a vesicular resting nucleus, the egg-nucleus, the 



