1896.] On Heredity and Rejuvenation. 91 



edity and development was stated in an entirely new form. 

 Since this publication of Nussbaum's we are seeking for the 

 explanation of the germinating power, and the propagation of 

 this power ; formerly we sought for the causes of the inheri- 

 tance of parental parts. The difference may be illustrated by 

 the following example. Before Nussbaum we were ruled by 

 Darwin's conception of Pangenesis, and we investigated ac- 

 cordingly for the agency by which the eye of the father re- 

 produced itself in the child. Since Nussbaum we leave Pan- 

 genesis behind — it belongs henceforth to the past — and try to 

 determine how the germinal substance behaves, and especially 

 in what way it is perpetuated from the ovum through the fol- 

 lowing developmental stages, so that it is finally still present 

 for the creation of the next generation. It is the conception 

 of the continuity of the germinal substance which we prize so 

 highly, and owe to Nussbaum. 



Larvae teach us that it cannot be special cells which affect 

 this continuity. In fact, we find the organs of larval life fully 

 differentiated before any sexual organs are recognizable, and 

 indeed, in the majority of known larvae we cannot recognize 

 even the rudiments of the sexual glands. On the contrary, 

 we find in larvae unmistakably differentiated locomotive appa- 

 ratus, such as cilia and often muscle fibres, a digestive canal, 

 sensory organs, and, in many cases, also special excretory or- 

 gans, and yet, only in a very few and exceptional cases can 

 we distinguish the cells which belong to the future sexual 

 glands. Thus, in regard to the primitive or larval type of de- 

 velopment, we cannot say that the germ cells are constantly 

 separated from the somatic cells during the segmentation of 

 the ovum, but must rather draw precisely the opposite conclu- 

 sion, namely, that the germ cells belong to the tissues which 

 arise latest. We often meet many tissues in larvae at a time 

 when there is still no indication of germ cells. We find the 

 same relations in embryos also, since in them the principal 

 tissues become recognizable before germ cells are present. 

 This fact was well established for vertebrates many years ago. 

 It is characteristic of Weismann that he long defended the 

 ■continuity of germ cells, in defiance of the facts. He has since 



