^1^ ZOOGENESIS ^^ 



subtraction or repression of certain features causing others to 

 stand out with special prominence. 



According to this hypothesis the most highly perfected animal 

 types are the protozoans, the coelenterates, and the vertebrates. 



In the radially symmetrical coelenterates the gastrula passes 

 with the least possible change into an adult form. As the adult 

 coelenterate represents a gastrula which is only slightly modified 

 and has lost relatively little of its inherent plasticity, we natu- 

 rally see in the coelenterates as a group such features as asexual 

 reproduction by budding and by fission, colony formation, di- 

 versification of polyps in a colony, reparation, sexual reproduction 

 by larvae, and variation in individual and colonial form developed 

 to an extent far beyond anything seen elsewhere in the animal 

 world. The persistence of features inherent in the gastrula 

 seems to be the only reasonable explanation for the extraordinary 

 diversity so characteristic of this enormous and important group 

 of animals. 



In the vertebrates all of the features inherent in the gastrula 

 are again present, but they are present in a very highly modified 

 and static form as a result of the extensive and intricate changes 

 which have taken place during the development of the indi- 

 viduals. The vertebrates therefore are so very delicately bal- 

 anced, both in respect to their internal mechanism and reactions 

 and in regard to their external contacts, that they have retained 

 none of the plasticity characteristic of the gastrula, or of the 

 coelenterates. They are incapable of asexual reproduction, of 

 colony formation, of extensive reparation of lost parts, or of as- 

 suming any great variety of form. 



Structurally the least perfect animal types are those included 

 in the Vermiformes, because each of these types lacks the maxi- 

 mum number of the features inherent in the gastrula. But the 

 sum total of the features found widely distributed in the Vermi- 

 formes as we know them would be essentially the equivalent of 

 the features found in the coelenterates. 



Such a concept of the development of animal life may be illus- 

 trated by comparing the primitive gastrula to a rosebud. As the 

 rosebud opens the petals unfold and grow out to their full size. 

 Each petal may be taken to represent a separate developmental 

 line running from the gastrula to one of the major groups, which 

 itself would be represented by the petal's outer edge. 



[2-59] 



