FOUNDATION OF A THEORY OF HEREDITY. 185 



nuclei therefore 'determine the specific direction in which an 

 organism developes.' 



The opinion derived from the recent study of the phenomena of 

 fertilization that the nucleus impresses its specific character upon 

 the cell, has received conclusive and important confirmation in the 

 experiments upon the regeneration of Infusoria, conducted simul- 

 taneously by M. Nussbaum 1 at Bonn, and by A. Gruber 2 at 

 Freiburg. Nussbaum's statement that an artificially separated 

 portion of a Paramaeciiim, 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 In- 

 fusoria alive for several days. Moreover, Gruber had previously 

 shown that individual Protozoa 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 con- 

 tain nuclei always regenerate. It is therefore only under the in- 

 fluence 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 the first segmentation nucleus contains, in its mole- 

 cular structure, the whole of the inherited tendencies of develop- 

 ment, it must follow that during segmentation and subsequent 

 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 depend, ceteris paridus, 

 upon identical nucleoplasm, and conversely different cells depend 

 upon differences in the nucleoplasm. The fact that the embryo 

 grows more strongly in one direction than in another, that its cell- 

 layers are of different nature and are ultimately differentiated 

 into various organs and tissues, forces us to accept the conclu- 

 sion that the nuclear substance has also been changed in nature, 

 and that such changes take place during ontogenetic development 



1 M. Nussbaum, ' Sitzungber. der Niederrheinischen Gesellschaft fur Natur- und 

 Heilkunde.' Dec. 15, 1884. 



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



