98 



THE FROG. 



of the polar body the black pole of the egg becomes slightly 

 flattened, leaving a space between the egg and the vitelline 

 membrane (Fig. 45, C) ; this space is occupied at once by a peri- 

 vitelline fluid, exuded from the egg, and in this fluid the polar 



uc 



FIG. 45. Successive stages in the maturation of the egg of the Frog. The 

 eggs are represented as bisected vertically, x 25. (After O. Schultze.) 



A, stage in which the nucleus has commenced to shrink, and the nuclear skein is 

 formed in its centre. B> stage in which the nuclear skein has moved to the surface of 

 the egg, just prior to the formation of the first polar body. C> stage in which the first 

 polar body has been formed, by division of the nuclear skein, and extruded. D. stage in 

 which the second polar body has been extruded, and the remaining part of the nuclear 

 skein, or female pronucleus, has retreated from the surface of the egg, and is about to 

 unite with the male pronucleus or head of the spermatozoon. 



PB, first polar body. PB', second polar body. TJF, female pronucleus. UG, egg 

 nucleus, or germinal vesicle. TJH, peri-vitelline fluid exuded from germinal vesicle. 

 TJM, male pronucleus. Z, vitelline membrane. 



body may be seen as a minute ovoidal white body, usually lying 

 in a small depression on the surface of the egg (Fig. 45, C, PB). 



