516 PATTEN— COOPERATION AS A FACTOR IN EVOLUTION. 



serial organs and concentric layers which prevail throughout the 

 entire series of segmented animals. 



Fig. 2. Diagram to illustrate the transition from the radial to the 

 apico-lateral type of growth, in an arachnid embryo growing on a spherical 

 yolk surface. 



A-C. Surface view showing relation of the gastrula stage (radial type) 

 to the apico-bilateral type; also the relation of marginal growth (concres- 

 cence), apical growth, blastopore, primitive mouth, and telopore, to one 

 another. 



D-G. Same, in transverse section. 



H-J. Longitudinal sections showing the four primary channels of ex- 

 change, nervous system, alimentary canal, heart, and coelom. 



With the growth of the film around the egg, the great landmarks 

 in the morphology of segmented animals may be definitely located: 

 namely, head and tail end, right and left sides, and neural and 

 haemal surfaces; and the three great channels of conveyance: nerve 

 cord, alimentary canal, and heart, with the segmented channels aris- 

 ing from them, are definitely established (Fig. 2). It might well 

 be said that these landmarks are as unmistakable as the north and 

 the south pole, or the right and left hand, if it were not for the fact 



