The Cellular Basis 209 



differential factors of heredity and development must be located 

 in the cytoplasm. Such factors would probably not be genes and 

 would not be transmitted in Mendelian fashion, -but they would 

 need to be present in the cytoplasm from the very beginnings of 

 ontogeny. They need not be numerous in fact they are prob- 

 ably few in number but they are absolutely indispensable to 

 development. If a few orientating differentiations such as 

 polarity and symmetry are present in the cytoplasm at the be- 

 ginning of ontogeny all other differentiations of development can 

 be explained as due to the interaction of non-differentiating 

 genes on different parts of this cytoplasm, but there is no mecharr- 

 ism by which embryonic differentiations could come from the 

 action of non-differentiating genes on a homogeneous cytoplasm. 

 The genes or Mendelian factors are undoubtedly located in the 

 chromosomes and they are sometimes regarded as the only dif- 

 ferential factors of development, but if this w"ere true these genes 

 would of necessity have to undergo differential division and dis- 

 tribution to the cleavage cells; since this is not true it must be 

 that some of the differential factors of development lie outside 

 of the nucleus and if they are inherited, as most of these early 

 orientations are, they must lie in the cytoplasm. 



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. In sexual reproduction 

 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 off- 

 spring but the germinal factors or causes are transmitted, and under 

 proper conditions of environment these give rise to developed char- 

 acters. Every oosperm as well as every developed organism differs 

 more or less from every other one and this remarkable condition is 



