178 THE PHILOSOPHY OF BIOLOGY 



lishment of this homology. The newer embryology 

 recognises the difficulties implied in the application, 

 in all its exclusiveness, of the Gastrea-theory to the 

 higher phyla of multicellular animals ; so that nowa- 

 days it has been necessary to abandon the notion of 

 the metazoan animal as being built up from these three 

 primary germ-layers. At the conclusion of segmenta- 

 tion, then, the embryo consists of a mass of cells 

 similar to each other in structure, but differing in fate 

 and in potency. Some of these cells are destined to 

 give rise to the integument, the nervous system, and 

 the sense-organs ; others become the skeleton and 

 musculature ; and others again the organs of digestion, 

 assimilation, and excretion. A primary arrangement 

 of these groups of cells into three layers is indeed set 

 up in many cases of development, but it is plain that 

 this arrangement is far from being an universal one. 

 Modern embryology shows in the clearest possible 

 manner that at the end of segmentation the embryo 

 consists of a group of cells each of which has normally 

 a different fate in subsequent development. What 

 precisely each cell will become depends on its position 

 with regard to the others. But each cell is capable 

 of becoming more than it normally becomes : its 

 potency is greater than its actual fate. If the normal 

 course of development is interrupted, a cell, which 

 would usually have given rise to a part of the skeleton, 

 may give rise to a part of the alimentary canal. The 

 cells of the developing embryo are autonomous. 



In the normal course of development most of the 

 cells existing at the end of segmentation give rise to 

 the " body " of the organism, undergoing differentia- 

 tion as they so develop. But a few embryonic cells 

 persist without structural modification throughout the 

 development of the animal. They divide and grow 



