110 HEREDITY AND ENVIRONMENT 



of every stage from the egg to the adult con- 

 dition. For every inherited character, whether 

 embryonic or adult, there is some germinal 

 basis. We receive from our parents germ cells 

 of a particular kind and constitution, and 

 under given conditions of environment these 

 cells undergo regular transformations and 

 differentiations in the course of development, 

 which differentiations lead to particular adult 

 characteristics. In the last analysis the causes 

 of heredity and development are problems of 

 cell structures and functions, problems of the 

 formation of particular kinds of germ cells, of 

 the fusion of these cells in fertilization, and of 

 the subsequent formation of the various types 

 of somatic cells from the fertilized egg cell. 



B. THE GERM CELLS 



Observations and experiments on developed 

 animals and plants have furnished us with a 

 knowledge of the finished products of inheri- 

 tance, but the actual stages and causes of in- 

 heritance, the real mechanisms of heredity, are 

 to be found only in a study of the germ 

 cells and their development. Although many 



