The American Naturalist. 



[i.t 



given up this wrong view and put in its place his hypothesis 

 of the continuity of germ plasm. Of Nussbaum's conceptions, 

 Weismann has left out the fruitful part, and has sown broad- 

 cast those ideas which were incapable of fruitful development. 

 He has attempted to defend his notion of the difference be- 

 tween the elements of the embryo destined for the construction 

 of the body, on the one hand, and those elements destined for 

 sexual propagation on the other. Now, since the sexual cells 

 usually develop from somatic cells, he was forced to assume 

 that there is a mysterious substance which he names " Keim- 

 plasma" This substance is supposed to store itself in the 

 body by some secret way, to separate itself at command from 

 the histogenic plasm, to appear unchanged and ready to be 

 the exclusive agent of hereditary transmission. 



Nussbaum furnished the conception of the continuity of 

 germinal substance, which appears to be of immeasurable im- 

 portance for the scientific investigation of the phenomena of 

 heredity. But this continuity holds for all cells which arise 

 from the fertilized ovum, as explained in the first section of 

 this article. We must, therefore, seek for the causes of the 

 differentiation of cells, that is to say, for the causes of the pro- 

 duction of nerve cells, muscle cells, gland cells, etc, and of the 

 production of germ cells. 



I will now try to make clear the significance of the compar- 

 ison between larv* and embryos for the interpretation of germ 

 cells. This calls for a short digression. 



In the course of my investigations on " Senescence and Re- 

 juvenation," of which only the first part has been published 

 (Journal of Physiology, xii, 97), I learned that as cells become 

 older there occurs an increase of the protoplasm in proportion 

 to the nucleus, and I further succeeded in proving, as an es- 

 sential process in reproduction, the formation of cells with 

 comparatively little protoplasm. Further, it was found prob- 

 able that a rapid multiplication of cells is only then possible 

 when the cells have small protoplasmatic bodies (Proc. A. A. 

 A. S., XXXIX (1890). We, therefore, have learned that the 

 power of development depends on a special condition of the 

 cell. By these facts I have been led directly to the following 

 hypothesis : 



