CHAPTER IV 

 INTERNAL FACTORS 



1. THE INITIAL STRUCTURE -OF THE GKERM AS 

 A CAUSE OF DIFFERENTIATION 



1. THE MODERN FORM OF THE PREFORMATIONIST DOCTRINE. 



As has been already pointed out, it was in the hands of Roux 

 that the principle of germinal localization first advanced by His 

 assumed the rank and importance of a theory of development. 

 On this ' Mosaik-Theorie ' of self-differentiation the precise 

 relation observable between the several parts of the embryo and 

 certain definite regions of the undeveloped germ is not merely 

 customary or normal, but necessary and causal. The germ-cell 

 is endowed with a preformed structure corresponding to the 

 structure of the organism which is to arise from it; each part 

 of this structure is predetermined for the formation of some 

 particular member of the embryo, and out of it no other member 

 can, under ordinary circumstances, be made. The causes of its 

 development, regarded as a specific activity of the organism, 

 as leading to the production of a form which is like that of the 

 parents which gave it birth, lie wholly within this pre-existing 

 structure, and of each part within each part. Although the 

 influence of the environment and, in later stages, of the parts 

 on one another is not entirely excluded, the factors on which 

 the differentiation of the whole and of each part depends are 

 essentially internal, and all that happens is that by a continued 

 process of cell-division the parts are separated from one another 

 and the structure thus made palpable and manifest. Cell-division 

 in the embryo is therefore qualitatively unlike. 



The supposed proof, by Pniiger's experiment with forcibly 

 inverted Frogs' eggs, of the complete ' isotropy ' or equivalence 

 of all parts of the cytoplasm, compelled Roux to locate the 



