202 Heredity and Environment 



Summary 



All the phenomena of life, including heredity 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 extremely numerous permutations in the distribution of 

 the chromosomes of the sex cells in maturation and fertilization. 

 Sex is an inherited character dependent, primarily, upon an alter- 

 native distribution of certain chrmosomes to the germ cells. 

 There is much evidence that the factors for all sorts of Mendelian 

 characters are associated with the chromosomes. The differen- 

 tiation of the oosperm into the developed organism is accom- 

 plished in part by the associations and dissociations of germinal 

 units which lead to the formation of new materials, and in part 

 by the segregation and localization of these in definite cells. 



Germ cells and probably all other kinds of cells are almost 

 incredibly complex. We know that former students of the cell 

 greatly underestimated this complexity and there is no reason to 

 suppose that we have fully comprehended it. What Darwin said 

 of the entire organism may now be said of every cell : It "is a mi- 

 crocosm — a little universe, formed of a host of self-propagating 

 organisms, inconceivably minute and numerous as the stars in 

 heaven." 



