l88 CONTINUITY OF THE GERM-PLASM AS THE [IV. 



The nuclei therefore ' determine the specific direction in which 

 an organism developes.' 



The opinion— derived from the recent study of the pheno- 

 mena of fertihzation — that the nucleus impresses its specific 

 character upon the cell, has received conclusive and important 

 confirmation in the experiments upon the' regeneration of In- 

 fusoria, conducted simultaneously by M. Nussbaum' at Bonn, 

 and b}' A. Gruber'^ at Freiburg. Nussbaum's statement that 

 an artificially separated portion of a Paraiuaeciuni, which does 

 not contain any nuclear substance, immediately dies, must not 

 be accepted as of general application, for Gruber has kept 

 similar fragments of other Infusoria alive for several days. 

 Moreover, Gruber had previousl}' shown that individual Pro- 

 tozoa occur, which live in a normal manner, and are yet without 

 a nucleus, although this structure is present in other individuals 

 of the same species. But the meaning of the nucleus is made 

 clear by the fact, published by Gruber, that such artificially 

 separated fragments of Infusoria are incapable of regeneration, 

 while on the other hand those fragments which contain nuclei 

 always regenerate. It is therefore only under the infiuence of 

 the nucleus that the cell substance re-developes into the full 

 type of the species. In adopting the view that the nucleus is 

 the factor which determines the specific nature of the cell, we 

 stand on a firm foundation upon which we can build with 

 security. 



If therefore tlie first segmentation nucleus contains, in its 

 molecular structure, the whole of the inherited tendencies of 

 development, it must follow that during segmentation and sub- 

 sequent cell-division, the nucleoplasm will enter upon definite 

 and varied changes which must cause the differences appearing 

 in the cells which are produced ; for identical cell-bodies de- 

 pend, ceteris paribus, upon identical nucleoplasm, and conversely 

 diflerent cells depend upon diflerences in the nucleoplasm. 

 The fact that the embr^'o grows more strongly in one direction 

 than in another, that its ccll-la^'ers are of different nature and 

 are ultimately differentiated into various organs and tissues, — 

 forces us to accept the conclusion that the nuclear substance 



' M. Nussbaum, ' Sitzungsber. der Niederrhcinischen Gesellschaft fiir 

 Natur- und Hcilkunde.' Dec. 15, 1884. 



- A. Gruber, ' Biologisches Centralblatt,' Bd. IV. No. 23, and V. No. 5. 



