ZOOGENESIS 



suitable food in sufEcient quantity and of defending itself against 

 aggression. 



The possible number of forms into which the materials avail- 

 able in the gastrula may be modeled so as to produce an animal 

 type capable of successful existence is undoubtedly limited. 

 Probably the major groups of animals as we know them represent 

 the full total. 



The reason for the assumption that the major groups of ani- 

 mals represent the entire range of structural possibilities is that 

 they arrange themselves in a very definite order in relation to each 

 other, and in this definite order there are no gaps. This order 

 seems to correspond in the animal world to the periodic system of 

 the chemical elements in the inorganic world. The key to this 

 order is furnished by the group Vermiformes. 



We have already pointed out that these creatures, which have 

 a symmetry partly radial but chiefly bilateral, may be considered 

 as having arisen from the line of geometrical development leading 

 to the radially symmetrical coelenterates as a result of the appear- 

 ance of developmental irregularities in the early embryonic stages 

 leading to the formation of a mainly bilateral adult. 



The developmental figure represented by the radially symmetri- 

 cal coelenterates and the Vermiformes (fig. B, p. 140) would be a 

 central spot, indicating the coelenterates, surrounded by a circle 

 of more or less detached spots each of which represents one of the 

 several groups of "worms." 



So far as the adults are concerned, there is no connection be- 

 tween the spots. But the same figure viewed from the aspect of 

 the development of the individuals in each of these types of ani- 

 mals would show a vertical axis running from the primitive 

 single cell upward through a series of geometrical embryonic 

 stages — the two, four, eight, sixteen and thirty-two celled stage, 

 etc., and the blastula and gastrula — to the adult coelenterate. 

 From various points in this vertical line, mostly at or near the 

 early gastrula stage, lines would branch off and run diagonally 

 upward and outward to each of the spots representing one of the 

 flatworm or roundworm types (fig. C, p. 146). 



In the circle of forms represented by the Vermiformes (fig. E, 

 p. ^54, outer circle) we see four distinct and widely different struc- 

 tural types, two or more of which, however, may occur in closely 

 related animals. These four structural types are the following. 



t49] 



