AUGUST WEIS MANN 343 



Infusoria are incapable of regeneration, while on the other hand 

 those fragments which contain nuclei always regenerate. It is there- 

 fore only under the influence of the nucleus that the cell substance 

 re-develops 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 molecu- 

 lar structure, the whole of the inherited tendencies of development, it 

 must follow that during segmentation and subsequent cell-division, the 

 nucleoplasm will enter upon definite and varied changes which must 

 cause the diflferences appearing in the cells which are produced ; for 

 identical cell-bodies depend, ceteris paribus, upon identical nucleoplasm, 

 and conversely diflferent cells depend upon difiFerences in the nucleo- 

 plasm. The fact that the embryo grows more strongly in one direc- 

 tion 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 conclusion that the nuclear substance has also been 

 changed in nature, and that such changes take place during ontogenetic 

 development in a regular and definite manner. This view is also held 

 by Strasburger, and it must be the opinion of all who seek to derive 

 the development of inherited tendencies from the molecular structure 

 of the germ-plasm, instead of from preformed gemmules. 



We are thus led to the important question as to the forces by which 

 the determining substance or nucleoplasm is changed, and as to the 

 manner in which it changes during the course of ontogeny, and on the 

 answer to this question our further conclusions must depend. The 

 simplest hypothesis would be to suppose that, at each division of the 

 nucleus, its specific substance divides into two halves of unequal qual- 

 ity, so that the cell-bodies would also be transformed ; for we have seen 

 that the character of a cell is determined by that of its nucleus. Thus 

 in any Metazoon the first two segmentation spheres would be trans- 

 formed in such a manner that one only contained the hereditary ten- 

 dencies of the endoderm and the other those of the ectoderm, and 

 therefore, at a later stage, the cells of the endoderm would arise from 

 the one and those of the ectoderm from the other ; and this is actually 

 known to occur. In the course of further division the nucleoplasm of 

 the first ectoderm cell would again divide unequally, e. g., into the 



