ON THE ANCESTRAL FORM OF THE CHORDATA. 325 



Till further light is thrown by fresh discoveries upon the primi- 

 tive condition of the posterior continuation of the vertebrate 

 alimentary tract, it is perhaps fruitless to attempt to work out 

 more in detail the above speculation. 



Body cavity and mesoblastic somites. The Chordata, or 

 at least the most primitive existing members of the group, are 

 characterized by the fact that the body cavity arises as a pair of 

 outgrowths of the archenteric cavity. This feature 1 in the de- 

 velopment is a nearly certain indication that the Chordata are a 

 very primitive stock. The most remarkable point with reference 

 to the development of the two outgrowths is, however, the fact 

 that the dorsal part of each outgrowth becomes separated from 

 the ventral. Its walls become segmented and form the meso- 

 blastic somites, which eventually, on the obliteration of their 

 cavity, give rise to the muscle-plates and to the tissue surround- 

 ing the notochord. It is not easy to understand the full 

 significance of the processes concerned in the formation of the 

 mesoblastic somites (vide p. 296). The mesoblastic somites 

 have no doubt a striking resemblance to the mesoblastic somites 

 of the Chaetopods, and most probably the segmentation of the 

 mesoblast in the two groups is a phenomenon of the same 

 nature ; but the difference in origin between the two types of 

 mesoblastic somites is so striking, and the development of the 

 muscular system from them is so dissimilar in the two groups, as 

 to render a direct descent of the Chordata from the Chaetopoda 

 very improbable. The ventral parts of the original outgrowth 

 give rise to the permanent body cavity, which appears originally 

 to have been divided into two parts by a dorsal and a ventral 

 mesentery. 



The notochord. The most characteristic organ of the 

 Chordata is without doubt the notochord. The ontosrenetic 



o 



development of this organ probably indicates that it arose as a 

 differentiation of the dorsal wall of the archenteron ; at the same 

 time it is not perhaps safe to lay too much stress upon its mode 

 of development. Embryological and anatomical evidence de- 

 monstrate, however, in the clearest manner that the early Chor- 

 data were provided with this organ as their sole axial skeleton ; 



1 Vide the chapter on the Germinal Layers. 



