GENERAL EMBRYOLOGY 



133 



r 



B 



C 



Formation of the Polar Bodies. The germinal vesicle initiates 

 these changes, its walls disappearing, its contents in 

 part mingling with the cytoplasm of the egg, in part 

 being employed in 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. 95, A). Now begins a regular 

 cell-division, 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. 

 95, C). The latter projects above the surface carry- 

 ing with it one half of the spindle, and when the globule 

 is cut off half of the spindle is included in it. 



The Second Polar Body. The part of the directive 

 spindle remaining in the egg immediately forms a new 

 spindle; the cell-budding is repeated (C, D) and leads 

 to the formation of the second polar body. As a result 

 two small cells (fig. 95, ) lie at one pole of the egg, 

 in many cases even three, since during the formation 

 of the second polar body the first may divide. The 

 part of the directive spindle remaining after the second 

 division becomes a vesicular resting nucleus, the egg- 

 nucleus or female pronuclcns, the characteristic feature 

 of the ripe egg capable of fertilization. In other words, 

 by a double division there have been formed from the 

 immature egg four (sometimes three) cells, of which 

 one has retained by far the greatest part of the original 

 mass of the cell and constitutes the ripe egg, while the 

 others are small bodies like rudimentary eggs. The 

 name directive corpuscles was given to them because 

 in very many cases their position renders possible a 

 definite orientation of the egg; i.e., a diameter, the 

 main axis, can be passed through the egg, one end of 

 which is marked by the directive corpuscles. With 

 reference to later processes of development this end is 

 called the animal pole of the egg, the opposite end the 

 vegetative pole. 



D 



FIG. 95- 

 Formation of polar 

 globules in AM iirix 

 megalocephala (dia- 

 grammatic, after 

 Boveri). .-1. first 

 directive spindle; 

 B, cutting off of 

 first polar body; C 

 and /', two stairs 

 of the second spin- 

 dle; E, separation 

 of second polar 

 body. 



Relation between Maturation and Fertilization. 



In many cases the maturation takes place before the entrance of the sperm, 

 either in the ovary or at the beginning of the oviduct; in many animals, on 



