IV.] FOUNDATION OF A THEORY OF HEREDITY. 211 



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 Nussbaum 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 a hypothesis. But 

 both these writers have worked out the idea in the form of a 

 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 Phanero- 

 gams, 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 predestined 

 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 \ I concluded 

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

 therefore invisible 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 pre- 

 viously 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 in- 

 dividuals 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 localized area from its original position (the 



^ Weismann, ' Die Entstehung der Sexualzellen bei den Hydro- 

 medusen.' Jena, 1883. 



P 2 



