190 THE GERM-PLASM 



cells lying in these tracks are alone capable of giving rise to 

 germ-cells, can hardly be explained otherwise than by assuming 

 that these cells alone contain germ-plasm along with their 

 special idioplasm. If germ-plasm could be produced from the 

 idioplasm of ordinary somatic cells, it would be impossible to 

 see why.germ-cells should not be formed in Hydroids in case of 

 need by the transformation of young ectoderm cells : but this 

 never happens. And even if we wished to assume that the 

 endoderm cells, as such, possessed an idioplasm which could not 

 be transformed into germ-plasm, while the nature of the ecto- 

 derm cells rendered such a transformation possible, this assump- 

 tion would be contradicted by other facts ; for, as far as we 

 know, the germ-cells arise exclusively from the endoderm cells 

 in the higher medusae, and in the polypes nearly related to 

 them. In this case therefore the germ-tracks are situated in 

 the endoderm, — that is to say, the germ-plasm is only passed 

 into certain series of cells in the endoderm, and the reserve 

 material of unalterable germ-plasm, which will serve for the 

 formation of the germ-cells, is passed into the primary endo- 

 derm cell only in the process of segmentation of the ovum, and 

 is handed on by it. In the Vertebrata the germ-cells become 

 differentiated from certain groups of mesoderm cells, and they 

 are never found in any other part of the body. In this case 

 the germ-track passes from the fertilised egg-cell into those 

 segmentation cells from which the primary cells of the whole 

 mesoderm are formed, in which latter it follows a closely con- 

 fined course. 



All these facts support the assumption that somatic idioplasm 

 is never transformed into germ-plasm, and this conclusion forms 

 the basis of the theory of the composition of the germ-plasm as 

 propounded here. It is obvious that its composition out of deter- 

 minants which gradually split up into smaller and smaller 

 groups in the course of ontogeny, cannot be brought into 

 agreement with the conception of the re-transformation of 

 somatic idioplasm into germ-plasm. If, as we have assumed, 

 each cell in the body only contains one determinant, the 

 germ-plasm — which is composed of hundreds of thousands 

 of determinants — could only be produced from somatic idio- 

 plasm if cells containing all the different kinds of determinants 

 which are present in the body were to become fused together 

 int(^ o)ie cell, their contained idioplasm likewise combining to 



