MATUKATION OF THE EGG. 



Concerning the real nature and significance of the changes 

 described above there has been much discussion, and the matter 



FIG. 1. Successive stages in the maturation of the egg of the Frog. The 

 eggs are represented as bisected vertically, x 25. (After 0. 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 formation of the first polar body. C> stage iu 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. UF, female pronucleus. TIG, egg 

 nucleus, or germinal vesicle. TTH, fluid exuded from germinal vesicle. TJM, male 

 pronucleus. Z, vitelline membrane. 



is not yet thoroughly understood. The points of chief impor- 

 tance appear to be the following. 



1. The process of maturation concerns the egg alone. It 



