^^ ZOOGENESIS ^l' 



lines of departure are accompanied by the appearance of meso- 

 derm between tlie ectodermal and endodermal layers of the 

 gastrula. 



I. In the ctenophores the body is divided into two similar 

 and equal halves by a plane passing through the long axis of the 

 flattened stomach, and also into two similar halves by a plane 

 at right angles to this which passes through the long axis of the 

 so-called funnel into which the stomach leads. Thus either line 

 in a right angled cross divides the ctenophore into two equal 

 halves. This very distinctive symmetry is characteristic of, and 

 is confined to, the ctenophores. 



1. In the Vermiformes the body is divided into equal and simi- 

 lar halves by a plane passing through the long axis of the body 

 dorsoventrally. But there is always a greater or lesser amount 

 of radial symmetry evident in the nervous system, the digestive 

 system, or the head structures. The Vermiformes are bilateral, 

 though always showing a certain amount of radial symmetry. 



sess radially symmetrical bodies composed of two layers of cells 

 which arise from an original single cell through regular geomet- 

 rical cell division. 



As there is no reason for assuming that irregular adhesion of 

 cells after division necessarily preceded regular geometrical adhe- 

 sion of cells after division, there are no grounds for supposing 

 that the coelenterates are not as old as the sponges or the proto- 

 zoans. 



The single celled animals or protozoans, the sponges, and the 

 two layered radially symmetrical animals each represent the 

 logical end product of a special type of cell division and of cell be- 

 havior after division (fig. A, p. ^38). There is not the slightest 

 evidence that any one of these animal types preceded the other 

 two in appearance. 



We now come to those animals in which the body is made up 

 of three layers of cells. The gastrula is formed as just described 

 (figs. 110-117, p. 185), but a greater or lesser number of cells, aris- 

 ing in various ways, make their appearance within the hollow 

 wall of the gastrula (compare figs. 116 and 117 with fig. 12.0, p. 

 185) and subsequently form the mesoderm. 



