CH. XII] INTERPRETATIONS AND CONCLUSIONS 125 



immediately resulting from the gastrula is, from the second 

 furrow on, a mosaic work of at least four vertical, independent 

 pieces. How far this mosaic work of four pieces is altered by 

 later changes in the position of the material, and by differentiat- 

 ing correlation, is not known. 



Roux also stated clearly the relation that exists between the 

 method of self-differentiation, and the method of interaction 

 of the parts on one another, and the bearing of these questions 

 on the older problems of evolution and epigenesis. If many 

 portions of the egg are differentiated owing to their inherent 

 power, and produce in this way the manifold differentiations 

 seen in the embryo, then the egg must have been composed 

 in the beginning of many parts bound up together, and the 

 development is a metamorphosis or an unfolding of its pecu- 

 liarities ; i.e. the development is an evolution. Further, the 

 cleavage not only divides the egg into smaller parts, but at 

 the same time localizes the differentiated material, so that this 

 material is arranged definitely in relation to later development. 

 This result appears possible only through a qualitative sepa- 

 ration of the material during the course of the cleavage. If 

 this is true, we see that the development depends on the 

 molecular structure of the egg, and therefore further analysis 

 is beyond our reach. The segmented egg would be then only 

 the sum of its independent parts, and during the period of the 

 self-differentiation of these parts, there has been no united 

 action to form a whole. Therefore the whole can have no 

 regulating or formative influence on the parts. 



If this view be true, His's principle of germinal localization 

 in the egg has not only a descriptive worth, but also expresses 

 a causal relation, so that organs can be referred to parts of 

 the fertilized egg, and even to the unfertilized egg. 1 



If. on the other hand, development takes place as a result 

 of the interaction of all or many parts on one another, then 

 the fertilized egg may be composed of a very few differentiated 

 parts, which by their interaction produce a greater and greater 



1 We could explain those exceptional cases in which two embryos arise from 

 one egg, if we supposed that after the first cleavage there was a sort of doubling, 

 in each blastomere, of the primary constituents of the body (Roux). 



