58 The Unity of the Organism 



the single matter of reference of the several parts to their 

 appropriate germ-layers. The work of these students car- 

 ries the subject into other fields, particularly those of re- 

 generation, and of formative stimulation. 



The Germ-Layer Theory and the Germ-Plasm Theory 



This discussion of the germ-layers will terminate with a 

 section on the part played by the layers in producing the sex- 

 cells where propagation is by the sexual method. This ter- 

 mination will also be the culmination in point of importance, 

 since it will lead as well into the examination, to be con- 

 tinued in other sections and chapters, of the results and 

 the general modes of reasoning of the Weismannian school of 

 speculation about heredity. 



The manner of involvement of the layers in the specula- 

 tions of this school becomes apparent on a moment's reflec- 

 tion. As is widely known, Weismann and his adlierents 

 conceive a particular substance known as germ-plasm to 

 which all the phenomena of heredity are due, this being fun- 

 damentally different from and independent of the great mass 

 of substance called by them somatoplasm which makes up 

 tlie bodies of organisms. Not only is this germ-plasm quite 

 apart from the somatoplasm in a given individual plant or 

 animal, but it passes along from parent to offspring, gen- 

 eration after generation, wholly uncontaminated, as one may 

 say, by contact with the somatoplasm. This supposition of 

 a propagative stream or string has been elaborated into 

 what is known as the doctrine of the "continuity of the 

 germ-plasm." A point fundamental to the doctrine is that 

 not merely the germ-plasm may pass over in this way from 

 parent to offspring, or that so7ne portion of it always does 

 thus pass, but that all the germ-plasm the offspring ever has 

 comes from this source. Otherwise expressed, the doctrine 

 is tliat germ-plasm is never produced anew in a strict sense. 



