68 The Unity of the Organism 



the following: "After all this, there can be no doubt that the 

 germ cells may reach their differentiation and separation 

 from somatic cells only when the germ-layers have long since 

 been formed, and it is impossible to accept as a general law 

 the view of Xus.sbaum that sex cells are 'absolutely indepen- 

 dent of the germ layers.' So far as we can now see, the sex- 

 cells always arise in the hydroids from elements of one of the 

 germ-layers and they are not merely inclusions in a germ- 

 Iryqi' but are derivatives, are division products of it." ^^ 

 Stripj^ed of all so})histry, how is it ])ossible to avoid seeing 

 that we have before us a clear case in which Weismann can 

 defend his doctrine of heredity at one of its most critical 

 points only })y making purely speculative considerations 

 supplant observational evidence which he himself produced 

 at an earlier period in his career? 



The conception of an "hereditary substance" distinct 

 from a non-hereditary substance, by whatever name called, 

 and continuous from parent to offspring is contrary to the 

 observed facts of sexual reproduction in the hydromedus^e 

 as established by Weismann himself and by other and later 

 biologists of unquestioned competency and trustworthiness. 

 To this conclusion we are forced by an examination of the 

 available knowledge of the sex-cells in their relation to the 

 germ-layers in this group of organisms. 



The Stronglij Oryanismal Implications, of Gocttc's Conclu- 

 sions on the Origin and Migration of Germ- 

 Cells in Hydroids 



With this conclusion we return to the examination of the 

 constructive as contrasted with the destructive results of 

 Goette's research. We have sliown the most specific and im- 

 mediate of these as viewed from the standpoint of this sec- 

 tion on the organism and its germ-layers, that, namely, 



