OX THE ANCESTRAL FORM OF THE CHORD AT A. 261 



probably the homologue of the olfactory organ of the Vertebrata ; 

 and it is quite possible that this bulb is the reduced rudiment of 

 what forms the fore-brain in the Vertebrata. 



The evidence at our disposal appears to me to indicate that the 

 third nerve belongs to the cranio-spinal series of segmental nerves, 

 while the optic and olfactory nerves appear to me equally clearly 

 not to belong to this series 1 . The mid-brain, as giving origin to the 

 third nerve, would appear not to have been part of the ganglion of 

 the praeoral lobe. 



These considerations indicate with fair probability that the part 

 of the head containing the fore-brain is the equivalent of the praeoral 

 lobe of many Invertebrate forms ; and the primitive position of the 

 Vertebrate mouth 011 the ventral side of the head affords a distinct 

 support for this view. It must however be admitted that this part 

 of the head is not sharply separated in development from that behind ; 

 and, though the fore-brain is usually differentiated very early as a 

 distinct lobe of the primitive nervous tube, yet that such differentia- 

 tion is hardly more marked than in the other parts of the brain. The 

 termination 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 appeal's 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 pari 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 Arthro- 

 pods; 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 ap- 

 pears however to have suffered in the course of its differentiation 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''. 



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, 



1 Marshall, in bis valuable paper on the development of the olfactory organ, takes 

 a very different view of this subject. For a discussion of this view I must refer the 

 reader to the chapter on the nervous system. 



2 The lateral branch of the vagus nerve probably became differentiated in connec- 

 tion 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). 



