HIGHWAY OF ANIMAL EVOLUTION 213 



There is apparently but one way to meet these basic 

 demands and thus overcome those narrower construc- 

 tive limitations so characteristic of the inferior types 

 of vital organization. That way is the triaxial (or 

 apico-quadrilateral) system of growth. 



In this constructive system, as in the other systems 

 we have considered, growth begins by the successive 

 divisions of an initial cell, or ovum; but the multi- 

 cellular body thus produced always grows unequally 

 in the three planes of space, producing a more or less 

 cylindrical body with six unlike sides, progressively 

 upbuilt on three right-angled, gradient axes. 



The two opposing ends and four opposing sides are 

 radically different in structure and function, and be- 

 come respectively head end and tail end, right and left, 

 neural and haemal sides. These basic architectural 

 and functional distinctions never change, and they can 

 be easily identified in all animals of this structural 

 type, down to the lowest invertebrates, to their earliest 

 embryonic stages, and in some cases to the egg itself, figs. 

 5 and 6. 



This architectural system meets the basic demands 

 of growth in ways that no other conceivable system can 

 do, more particularly in the following respects: (i) 

 It may be permanently and profitably oriented to the 

 three great systems of world action in which it lives; 

 first and foremost, to gravity, the one universal, direct 

 acting constant in the world at large; to light, second 

 only to gravity in its straight line action and the con- 

 stancy of its source, even though the angles of incidence 

 may vary, and the flow be intermittent; and finally to 

 the great terrestrial, tangential planes in which are va- 

 riously distributed, or move in horizontal directions, 



