FOUNDATION OF A THEORY OF HEREDITY. 207 



cannot be identical with that of the parent germ-cell. In order to 

 prove this, it is only necessary to refer to- the arguments as to the 

 ontogenetic stages of the idioplasm. In the above-mentioned 

 instances, the continuity from the germ-substance of the parent 

 to that of the offspring can only be explained by the supposition 

 that the somatic nucleoplasm still contains some unchanged germ- 

 plasm. I believe that the fundamental idea of Jager and Nuss- 

 baum is quite correct : it is the same idea which has led me to the 

 hypothesis of the continuity of the germ-plasm, viz., the conviction 

 that heredity can only be understood by means of such an hypothesis. 

 But both these writers have worked out the idea in the form of an 

 hypothesis which does not correspond with the facts. That this is 

 the case is also shown by the following words of Nussbaum ' the 

 cell-material of the individual (somatic cells) can never produce a 

 single sexual cell.' Such production undoubtedly takes place, not 

 only in Hydroids and Phanerogams, but in many other instances. 

 The germ-cells cannot indeed be produced by any indifferent cell 

 of embryonic character, but by certain cells, and under circumstances 

 which allow us to positively conclude that they have been pre- 

 destined for this purpose from the beginning. In other words, 

 the cells in question contain germ-plasm, and this alone enables 

 them to become germ-cells. 



As a result of my investigations on Hydroids l , I concluded that 

 the germ-plasm is present in a very finely divided and therefore in- 

 visible state in certain somatic cells, from the very beginning of 

 embryonic development, and that it is then transmitted through 

 innumerable cell-generations, to those remote individuals of the 

 colony in which sexual products are formed. This conclusion is 

 based upon the fact that germ-cells only occur in certain localized 

 areas ('Keimstatten') in which neither germ-cells nor primitive 

 germ-cells (the cells which are transformed into germ-cells at a later 

 period) were previously present. The primitive germ-cells are also 

 only formed in localized areas, arising from somatic cells of the 

 ectoderm. The place at which germ-cells arise is the same in all 

 individuals of the same species ; but differs in different species. It 

 can be shown that such differences correspond to different phyletic 

 stages of a process of displacement, which tends to remove the 



1 Weismann, ' Die Entstehung der Sexualzellen bei den Hydromedusen.* Jena, 

 1883. 



