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 crwra 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 cesophageal tube is found in the invertebrate, the infundibular 
tube exists in the vertebrate, so that if the words infundibular and 
cesophageal 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 cesophageal 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 
cesophagus 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 esophagus, 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, 7.e. esophagus, 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. 
