I 86 THE GERM-PLASM 



as indicating that the series has been developed in the reverse 

 direction, the late differentiation of the germ-cells being the 

 primary condition and the earlier separation of the two parts 

 then having arisen gradually in individual cases. There can 

 hardly be any doubt, indeed, that the early differentiation of the 

 germ-cells of the Diptera and Daphnidae is of a secondary 

 nature ; and it will presently be shown that in the case of 

 Hydroids such a shifting of the formative areas (' Bildungs- 

 statte") of the germ-cells — /.6'., the fact of their earlier differen- 

 tiation — can be actually proved. But the facts which have been 

 stated still support the interpretation of them given above, in so 

 far as they show that the germ-cells are by no means formed at 

 the time and in the place where they are actually wanted, and 

 that the time of their formation, in fact, varies very much, and 

 must have been changed in the course of phylogeny. The 

 direction in which this shifting originally took place — that 

 is to say, whether it proceeded from the egg to the close of on- 

 togeny or in the reverse direction — must be decided when our 

 knowledge of the facts is more complete. 



We might here lay stress on the fact that the destruction of 

 the sexual glands in an animal, however low in the scale, is not 

 followed by the formation of sexual cells in any other part of the 

 body. Castration might be expected to have this effect if germ- 

 cells could be formed from any young cells of the body. But 

 just as in the case of any other highly specialised organs, such 

 as the liver, kidneys, and central portions of the nervous system 

 in Vertebrates, such a replacement never occurs. This fact is 

 to be explained according to our present view by supposing that 

 the formation of these latter organs anew is impossible, because 

 the determinants necessary for such a development are not 

 present in any other cells of the body. The same conclusion 

 will, it seems to me, be inevitable in the case of the germ-cells ; 

 the idioplasm necessary for the format ion of germ-cells — i.e., 

 germ-plasm — must be absent in these cases, and germ-plasm at 

 any rate caiuiot be formed from somatic idioplasm. 



The case of the Hydroids * is probably still more convincing, 

 for here a natural shifting of the place of origin of the germ-cells 

 has actually taken place. As has already been mentioned, the 

 germ-cells of Hydroids first arise very late in the life-cycle, 



* Weismann, ' Die Enstehung der Sexualzellen bei den Hydromedusen,' 

 Jena, 1883. 



