76 GENERAL EMBRYOLOGY 



regarding the continuity of the chromosomes, an outcropping of the 

 opposed preformational and epigenetic conceptions, which pervade all 

 descriptions of developmental phenomena. The hypothesis of chromo- 

 somal continuity is essentially a preformational view; those who deny 

 continuity assume the epigenetic view. 



Perhaps the immediate solution of the difficulty here may not be un- 

 like the solution of the greater problem and more fundamental difference 

 of opinion. It seems likely that what is directly continuous from 

 nucleus to nucleus, i.e., what is preformed, is some sort of fundamental 

 organization determining the chromosomal structure of the nucleus, 

 just as the "organization" of the egg determines the structure of the 

 embryo developed from it. Yet what appears is a new structure, formed 

 epigenetically under or through the influence of the "organizational" 

 factor, by the material present, and subject to the modifying influences 

 of changing conditions external to the nucleus. That is to say, the 

 formation of the chromosomes out of the chromatin of the vegetative 

 nucleus is to be regarded as a true process of development. The reac- 

 tion between the fundamental organized structure of the nucleus, and 

 the stimuli acting upon it, consists in the formation of the chromosomes 

 and other structures, not to be seen in the nucleus, previous to this 

 reaction. The chromosomes are thus no more genetically continuous 

 than the organs of adults, and yet there is a real continuity of 

 organization underlying. 



In many respects the nucleus is analogous to an organism, the chro- 

 mosomes and other nuclear structures representing the organismal 

 organs; both are functionally specific, are constant in number, form, 

 and size throughout the species; both reproduce and exhibit develop- 

 ment as a form of response. As Wilson points out, the analogy is far 

 from complete no complete analogy is known, but that there is an 

 underlying organization of some kind, continuous and specific, seems 

 clear although we remain entirely ignorant of what it really is, and just 

 how it operates and is affected by new conditions. 



Before leaving the subjects of the cell and cell division we must con- 

 sider briefly two other questions which are of great importance, but 

 which are also still in a hypothetical state. These are, the nature of 

 the causes leading to cell division, and the nature of the fundamental 

 mechanism of the process. The interactions between the nucleus and 

 cytoplasm, and between these and the external medium, which consti- 

 tute the life of the cell, are largely, probably wholly, of a physico-chem- 

 ical nature. As such their normal procedure is dependent upon certain 

 mass relations of the interacting substances, and upon the maintenance 

 of adequate pathways of interaction between them. For these reasons 

 we look quite naturally, in searching for a possible cause of cell division, 

 to the volumetric relations of the nucleus and cytoplasm, and to the 



