OiV THE ANCESTRAL FORM OF THE CHORD AT A. 269 



post-anal gut {vide fig. 28*, p. 48), may have been the common cavity 

 into which both neural and alimentary tubes opened'. Till further 

 light is thrown by fresh discoveries upon the primitive 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 charac- 

 terized by the fact that the body-cavity arises as a pair of outgrowths 

 of the archenteric cavity. This featvire*^ in the development 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 out- 

 growth becomes separated from the ventral. Its walls become 

 segmented and form the mesoblastic somites, which eventually, on 

 the obliteration of their cavity, give rise to the muscle-plates and to 

 the tissue surrounding the notochord. It is not easy to under- 

 stand the full significance of the processes concerned in the forma- 

 tion of the mesoblastic somites {vide p. 246). The mesoblastic somites 

 have no doubt a striking resemblance to the mesoblastic somites 

 of the Chaitopods, and most probably the segmentation of the 

 mesoblast in the two groups is a phenomenon of the same nature; 

 but the difference in oi'igin 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 Cha^topoda 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 ontogenetic 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. Embryo- 

 losfical and anatomical evidence demonstrate, however, in the clearest 



o ... 



manner that the early Chordata were provided with this organ as their 

 sole axial skeleton ; and no invertebrate group can fairly be regarded 

 as genetically related to the Chordata till it can be shewn to possess 

 some organ either derived from a notochord, or capable of having 



1 As pointed out in Vol. i. p. 211, there is a striking similarity between the history 

 of the neurenteric canal in Vertebrates, and the history of the blastopore and ventral 

 groove as described by Kowalevsky in the larva of Chiton. Mr A. Sedgwick has 

 pointed out to me that the ciliat*^d ventral groove in Protoneomeuia, which contains 

 the anus, is probably the homologue of the groove found in the larva of Chiton, and 

 not, as nsuaUy supposed, simply the foot. Were this groove to be converted into a canal, 

 on the sides of which were placed the nervous cords, there would be formed a precisely 

 similar neurenteric canal to that in Vertebrata, though I do not mean to suggest that 

 there is any homology between the two {vide Hubrecht, Zool. Anzeiger, 1880, p. 589). 



- Vide the chapter on the Ciermiual Layers. 



