ZOOGENESIS 



a very early embryonic stage, would in the adult form give no 

 certain indication of their origin. They would appear as isolated 

 animal types with no close relatives. 



Only in this way may the various types included in the Vermi- 

 formes be explained. They may be interpreted as having arisen 

 simultaneously with the coelenterates through deviations from 

 the regular geometrical course of embryonic development leading 

 to a radially symmetrical adult. 



This deviation seems to have taken place at a slightly later 

 stage in some than in others, and furthermore it seems to have 

 been of several diverse types resulting in a curiously heterogene- 

 ous assemblage of forms relatively few of which are sufficiently 

 well balanced to be capable of independent, that is, non-parasitic 

 existence. 



None of the Vermiformes can be assumed to have been the 

 ancestors of anything else. As animal forms they are quite 

 unique. But taken as a whole they furnish a most significant 

 clue to the probable relationships of the other animal types. 



No matter how different they may be — whether clam, bird, 

 starfish, jointed worm, or other form of animal life — all of the 

 more complexly organized animals in the course of their develop- 

 ment pass through a gastrula or comparable stage. This gastrula 

 stage is the last stage common to them all. Since they all pass 

 through a gastrula stage they are all referable, as far as the gas- 

 trula, to a developmental line which, followed to its logical end, 

 leads to a radially symmetrical animal (fig. A, p. 138). 



Following — or usually during — the gastrula stage they all 

 diverge in different directions. The radial symmetry of the 

 gastrula is completely lost, and subsequent development leads 

 in all cases to the formation of a bilaterally symmetrical creature. 



All of the more complex animals agree in being bilaterally 

 symmetrical, and also in having the body derived from three germ 

 layers. Each major group, however, possesses characteristic 

 and distinctive features which have their origin in the very 

 varied behavior of the three germ layers — of themselves and in 

 reference to the other two — in late gastrula and subsequent 

 development. 



Thus a mollusk is a mollusk, a vertebrate is a vertebrate, an 

 echinoderm is an echinoderm, a jointed worm or annelid is an 

 annelid, and so on, in the late gastrula, or at the furthest imme- 



[M7] 



