CHAPTER II 

 POLAR BODIES AND FERTILIZATION 



Whether the egg leaves the ovary by means of its own 

 activity, or by some other mechanism, we do not know. That 

 the egg itself takes some part in the process seems possible 

 from the fact that it is set free only at a particular moment 

 of its maturation, i.e. at a time when certain processes have 

 taken place in its interior. This same process takes place 

 simultaneously in all the eggs in the ovary. The separation of 

 the egg from the ovary is not dependent upon the act of copu- 

 lation, for several cases are on record in which isolated females 

 were found to have eggs in the body-cavity and oviducts. 



The egg set free in the coelomic cavity is covered by an ex- 

 tremely thin membrane, the egg-membrane or vitelline mem- 

 brane. The egg itself is very soft and easily broken if handled. 

 Later, when in the oviduct, the protoplasm seems to become 

 more firm. 



The egg shows a white and a dark hemisphere. The relative 

 distribution of superficial pigment in the egg determines the 

 extent of the white and dark surfaces. The outer layer of pig- 

 ment in the black hemisphere seems to be in close contact with, 

 or fixed to, the vitelline membrane, but the pigment lying in 

 the protoplasm beneath the outer layer is free to move with any 

 movement of the protoplasm (Figs. 8, 9). The relative ex- 

 tent of surface of the egg that is black or white is variable in 

 different species, and even in different females of the same 

 species ; but all the eggs from one female show approximately 

 the same distribution of pigment. 



Extrusion of the First Polar Body and Egg-laying 



Just prior to the extrusion of the egg into the body-cavity of 

 the frog, the nucleus undergoes a remarkable change, so that in 



15 



