2 • OVULATION: ITS CONTROL BY 

 HORMONES AND LIGHT 



I he 



he following chapters will enlarge on the general outline of the prob- 

 lems of embryology already presented. We shall begin with a discussion 

 of the process of the release of eggs from the ovary, ovulation ; proceed with 

 a study of the characteristic organization of the egg as it comes from the 

 adult; and then study the process of fertilization, which stimulates the egg 

 to development. After this we shall undertake an analysis of the factors 

 operating during the formation of the various structures in the embryo. 

 Finally, the problem of growth and the control exercised by genes will con- 

 clude our discussion. In each case a short description of the process under 

 consideration will be given, and this will be followed by a step-by-step analysis 

 of the factors control] ing the process. 



Origin of the germ cells 



Cells which finally give rise to eggs and sperm are called germ cells. 

 According to some observations they first appear in the embryo as large, 

 single, ameboid cells and possess remarkable powers of migration. In the 

 chick they are said to arise from areas outside the embryo, to enter the embryo 

 by means of the blood vessels, and finally to leave the blood vessels and enter 



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