no TEE POPULAR SCIENCE MONTHLY 



development it is one and the same organism ; the egg is not one being 

 and the embryo another and the adult a third, but the egg of a human 

 being is a human being in the one-celled stage of development, and the 

 characteristics of the adult develop out of the egg and are not in some 

 mysterious way grafted upon it or transmitted to it. 



Parents do not transmit their characters, but their germ cells, to 

 their offspring, which germ cells in the course of long development give 

 rise to adult characters similar to those of the parents. The thing 

 which persists more or less completely from generation to generation is 

 the organization of the germ cells which differentiate in similar ways 

 in successive generations if the extrinsic factors of development remain 

 similar. 



In short, heredity may he defined as the particular germinal organi- 

 zation which is transmitted from one generation to the next: inheritance 

 or heritage is the sum of all those qualities which are determined or 

 caused by this germinal organization. Development is progressive and 

 coordinated differentiation of this germinal organization, by which it is 

 transformed into the adult organization. Differentiation is the forma- 

 tion and localization of many different Jcinds of substances out of the 

 germinal substances, of many different structures and functions out of 

 the relatively simple structures and functions of the oosperm. 



This germinal organization influences not merely adult characters, 

 but also the character of every stage from the egg to the adult condition. 

 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 environ- 

 ment 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 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 of their 

 development. Although many phenomena of inheritance have been dis- 

 covered 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 formulate theories of heredity to account for the 

 facts, but these theories are only temporary scaffolding to bridge the 

 gaps in our knowledge, and if we knew all that could be known about 



