14 THE ORIGIN OF VERTEBRATES 



portion, known as the third ventricle, a narrow tube passes to the 

 ventral surface of the brain. This tube is called the infundibulum, 

 and, extraordinary to relate, lies just anteriorly to the exits of the 

 third cranial or oculomotor nerves ; in other words, it marks the 

 termination of the series of spinal and cranial segmental nerves. 

 Further, on each side of this infundibular tube are lying the two 

 thick masses of the crura cerebri, the strands of fibres which connect 

 the higher brain-region proper with the lower region of the medulla 

 oblongata and spinal cord. Not only, then, are the nerve-masses 

 in the two systems exactly comparable, but in the very place where 

 the oesophageal tube is found in the invertebrate, the infundibular 

 tube exists in the vertebrate, so that if the words infundibular and 

 oesophageal are taken to be interchangable, then in every respect 

 the two central nervous systems are comparable. The brain proper 

 of the vertebrate, with its olfactory and optic nerves, becomes the 

 direct descendant of the supra-cesophageal ganglia ; the crura cerebri 

 become the oesophageal commissures, and the cranial and spinal 

 segmental nerves are respectively the nerves belonging to the infra- 

 cesophageal and ventral chain of ganglia. 



This overwhelmingly strong evidence has always pointed directly 

 to the origin of the vertebrate from some form among the segmented 

 group of invertebrates, annelid or arthropod, in which the original 

 oesophagus had become converted into the infundibulum, and a new 

 mouth formed. So far, the position of this school of anatomists was 

 extremely sound, for it is impossible to dispute the facts on which 

 it is based. Still, however, the fact remained that the gut of the 

 vertebrate lies ventrally to the nervous system, while that of the 

 invertebrate lies dorsally ; consequently, since the infundibulum was 

 in the position of the invertebrate oesophagus, it must originally have 

 entered into the gut, and since the vertebrate gut was lying ventrally 

 to it, it could only have opened into that gut in the invertebrate stage 

 by the shifting of dorsal and ventral surfaces. From this argument 

 it followed that the remains of the original mouth into which the in- 

 fundibulum, i.e. oesophagus, opened were to be sought for on the dorsal 

 side of the vertebrate brain. Here in all vertebrates there are two 

 spots where the roof of the brain is very thin, the one in the region of 

 the pineal body, and the other constituting the roof of the fourth ven- 

 tricle. Both of these places have had their advocates as the position of 

 the old mouth, the former being upheld by Owen, the latter by Dohrn. 



