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 infundibulum, _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 ventricle. 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. {15}The discovery that the pineal body was originally an eye, or, rather, a pair of eyes, has perhaps more than anything else proved the impossibility of accepting this reversal of surfaces as an explanation of the genesis of the vertebrate from the annelid group. For whereas a pair of eyes close to the mid-dorsal line is not only likely enough, but is actually found to exist among large numbers of arthropods, both living and extinct, a pair of eyes situated close to the mid-ventral line near the mouth is not only unheard of in nature, but so improbable as to render impossible the theory which necessitates such a position. Yet this very discovery gives the strongest possible additional support to the close identity in the plan of the central nervous system of vertebrate and appendiculate. A truly paradoxical situation! The very discovery which may almost be said to prove the truth of the hypothesis, is the very one which has done most to discredit it, because in the minds of its authors the only possible solution of the transition from the one group to the other was by means of the reversal of surfaces.