ON THE ANCESTRAL FORM OF THE CHORDATA. 315 



usually differentiated very early as a distinct lobe of the 

 primitive nervous tube, yet that such differentiation is hardly 

 more marked than in the other parts of the brain. The termi- 

 nation of the notochord immediately behind the fore-brain is, 

 however, an argument in favour of the morphological distinctness 

 of the latter structure. 



The evidence at our disposal appears to indicate that the 

 posterior part of the head was not differentiated from the trunk 

 in lower Chordata ; but that, as the Chordata rose in the scale 

 of development, more and more centralizing work became 

 thrown on the anterior part of the nervous cord, and part passu 

 this part became differentiated into the mid- and hind-brain. 

 An analogy for such a differentiation is supplied in the compound 

 subcesophageal ganglion of many Arthropods ; and, as will be 

 shewn in the chapter on the nervous system, there is strong 

 embryological evidence that the mid- and hind-brains had 

 primitively the same structure as the spinal cord. The head 

 appears however to have suffered in the course of its diffe- 

 rentiation a great concentration in its posterior part, which 

 becomes progressively more marked, even within the limits of 

 the surviving Vertebrata. This concentration is especially shewn 

 in the structure of the vagus nerve, which, as first pointed out 

 by Gegenbaur, bears evidence of having been originally composed 

 of a great series of nerves, each supplying a visceral cleft. 

 Rudiments of the posterior nerves still remain as the branches 

 to the oesophagus and stomach 1 . 



The atrophy of the posterior visceral clefts seems to have 

 taken place simultaneously with the concentration of the neural 

 part of the head ; but the former process did not proceed so 

 rapidly as the latter, so that the visceral region of the head is 

 longer in the lower Vertebrata than the neural region, and is 

 dorsally overlapped by the anterior part of the spinal cord and 

 the anterior muscle-plates (vide fig. 47). 



On the above view the posterior part of the head must have 

 been originally composed of a series of somites like those of the 



1 The lateral branch of the vagus nerve probably became differentiated in 

 connection with the lateral line, which seems to have been first formed in the 

 head, and subsequently to have extended into the trunk (vide section on Lateral 

 Line). 



