206 GRAND STRATEGY OF EVOLUTION 



ment for its members as regards contact, pressure, dis- 

 tribution of materials, and the nature of the stimuli 

 received from within and without. 



Such cells acquire, therefore, a more constant, or 

 fixed, relation to one another, and definite physical and 

 chemical conditions are established in and about them, 

 which henceforth determine many of the things they 

 may or may not do, and what they must do. Hence 

 cells, which may be potentially alike, when separated, 

 must become an organized society of different kinds 

 of cells, if united. Otherwise they must disband, or 

 never could have united. 



X. Radial Structural Plan (Goelenterates, or 

 Jelly-fish) 



If we assume that a certain group of cells is grow- 

 ing in a homogeneous medium, or environment, and 

 that the increase in numbers is approximately equal 

 in the three planes of space, spherical bodies will be 

 formed, in which unlike conditions for cell life appear 

 along concentric rings and radii, in accordance with 

 the geometric properties of spherical bodies. As a re- 

 sult of these local conditions, special kinds of cells 

 should arise locally, such as muscles, nerves, and sense 

 organs, arranged in more or less regular geometric pat- 

 terns, fig. 3. And this actually does take place in all the 

 coelenterates (jelly-fish) ; and something similar to it 

 in the corresponding stage of growth in all the higher 

 organisms (gastrulation and germ layers, ectoderm, 

 mesoderm, entoderm). 



But a symmetrical sphere, if it is ever formed, does 

 not persist. The aggregate always presents some 



