KEPKODUCTION AND HEREDITY 301 



forms and become co-ordinate, until finally a new organism has 

 been produced resembling that from which the germ-cells origi- 

 nated. How can we explain this mysterious process ? 



As far back as 1868 Darwin promulgated his ' Provisional 

 Hypothesis of Pangenesis ' to which he was led merely by 

 speculative considerations and entirely without a knowledge of 

 all the delicate processes of which we now know. In the opinion 

 of Darwin it is incorrect to say that the whole organism repro- 

 duces ; it is rather each individual cell or cell-species which repro- 

 duces itself : the nerve-cells reproduce nerve-cells, the muscle- 

 cells produce muscle-cells, the cells of the blood, blood-cells, 

 and so forth. The reproduction of the cells consists in secreting 

 minute germs which become distributed throughout the entire 

 body, finally to settle down in the germ-cells which are them- 

 selves devoid of character and undifferentiated. These minute 

 germs possess great independence and are able to multiply by 

 fission, producing continuously equivalent descendants. If now 

 such germ reaches an embryonic non-differentiated cell it may 

 force it into a definite developmental direction. If, for instance, 

 the little germ of a nerve-cell invades an embryonic cell this 

 becomes a nerve-cell. The more heterogeneous germs an embryo- 

 cell contains, the richer are its developmental possibilities. As 

 the reproductive cells are able to produce out of themselves 

 the complete organism, they must of necessity contain at least 

 one minute germ of each kind of body-cell. These minute germs 

 are able to remain in a state of inactivity for a considerable 

 time, sometimes throughout several generations, suddenly to 

 proceed once more to reproduction. This explains the appearance 

 of atavism. If the development is to take place normally, it is, 

 of course, necessary that all these minute germs awaken into 

 activity in proper succession. 



We see, therefore, that Darwin had already reached the 

 opinion that each heritable characteristic of the paternal 

 organism is bound to a specific material carrier, and must have 

 been deposited in the germ-cells. This essential point in 

 Darwin's doctrine is valid to-day. 



Our observations concerning mitotic nucleus-division, 

 maturation-division, and fertilization have led us to the con- 

 clusion that the chromatin of the nucleus in its totality is the 



