The Cellular Basis 133 



Definitions. In short, heredity may be defined as the continuity 

 from generation to generation of certain elements of germinal 

 organization. Heritage is the sum of all those qualities whiah are 

 determined or caused by this germinal organization. Develop- 

 ment is progressive and coordinated differentiation of the 

 oosperm, under the joint influence of heredity and environment, 

 by which it is transformed into the adult organisation. Differ*- 

 entiation is the formation and localisation of many different kinds 

 of substances out of the germinal substance, of many different 

 structures and functions out of the relatively simple structures 

 and functions of the oosperm. 



This germinal organization influences not merely adult charac- 

 ters but also the characters of every stage from the egg to the adult 

 condition. For every inherited character, whether embryonic or 

 adult, there is some germinal basis. 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 

 inheritance, but the actual stages and causes of inheritance, the 

 real mechanisms of heredity, are to be found only in a study of 

 the germ cells and their development. Although many phenomena 

 of inheritance have been discovered in the absence of any definite 

 knowledge of the mechanism of heredity, a scientific explanation 

 of these phenomena must wait upon the knowledge of their causes. 

 In the absence of such knowledge it has been necessary to formu- 

 late theories of heredity to account for the facts, but these theo- 

 ries are only temporary scaffolding to bridge the gaps in our 

 knowledge, and if we knew all that could be known about the 



