186 HEREDITY AND ENVIRONMENT 



the mitotic movements within the cell by the 

 persistence of the nuclear membrane during 

 division. In general the degree of differentia- 

 tion may be measured by the degree of un- 

 likeness betAveen different cells, and by the 

 completeness with which the protoplasm of 

 different cells is kept from intermingling. 



Summary 



All the phenomena of life, including hered- 

 ity and development, are cellular phenomena 

 in that they include only the activities of 

 cells or of cell aggregates. The cell is the 

 ultimate independent unit of organic structure 

 and function. The only living bond between 

 one generation and the next is found in the 

 sex cells and all inheritance must take place 

 through these cells. Inherited traits are not 

 transmitted from parents to offspring but the 

 germinal factors or causes are transmitted, 

 and under proper conditions of environment 

 these give rise to developed characters. Every 

 oosperm as well as every developed organism 

 differs more or less from every other one and 

 this remarkable condition is brought about by 



