108 HEREDITY AND ENVIRONMENT 



such as chromosomes, chromomeres, plasto- 

 somes and microsomes, and that with every 

 great improvement in the microscope and in 

 microscopical technique other structures are 

 made visible which were invisible before, and 

 whether the particular hypothetical units just 

 named are invisible or not seems to be a matter 

 of no great importance, seeing that, so far as 

 the analysis of the microscope is able to go, 

 there are in all protoplasm differentiated 

 units which are combined into a system; in 

 short, there is organization. 



5. Heredity and Development. — The germ 

 cells are individual organisms and after the 

 fertilization of the egg the new individual thus 

 formed remains distinct from every other one. 

 Furthermore, from its earliest to its latest 

 stage of 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 char- 

 acteristics of the adult develop out of the egg 

 and are not in some mysterious way grafted 

 upon it or transmitted to it. 



