It is possible that the fourth nerve and the superior oblique muscle of the eye which it supplies may be the last remaining remnants of a second præmandibular segment originally situated between the segment of the third nerve and that of the fifth nerve (mandibular segment). The conclusions arrived at may be summed up as follows: The relations which exist in all jaw-bearing Vertebrates between the mandibular arch and the oral aperture are secondary, and arose _pari passu_ with the evolution of the jaws[111]. [111] I do not mean to exclude the possibility of the mandibular arch having supported a suctorial mouth before it became converted into a pair of jaws. [FIG. 196. THE HEADS OF ELASMOBRANCH EMBRYOS AT TWO STAGES VIEWED AS TRANSPARENT OBJECTS. A. Pristiurus embryo of the same stage as fig. 28 F. B. Somewhat older Scyllium embryo. _III._ third nerve; _V._ fifth nerve; _VII._ seventh nerve; _au.n._ auditory nerve; _gl._ glossopharyngeal nerve; _Vg._ vagus nerve; _fb._ fore-brain; _pn._ pineal gland; _mb._ mid-brain; _hb._ hind-brain; _iv.v._ fourth ventricle; _cb._ cerebellum; _ol._ olfactory pit; _op._ eye; _au.V._ auditory vesicle; _m._ mesoblast at base of brain; _ch._ notochord; _ht._ heart; _Vc._ visceral clefts; _eg._ external gills; _pp._ sections of body cavity in the head.] The cranial flexure and the form of the head in vertebrate embryos. All embryologists who have studied the embryos of the various vertebrate groups have been struck with the remarkable similarity which exists between them, more especially as concerns the form of the head. This similarity is closest between the members of the Amniota, but there is also a very marked resemblance between the Amniota and the Elasmobranchii. The peculiarity in question, which is characteristically shewn in fig. 196, consists in the cerebral hemispheres and thalamencephalon being ventrally flexed to such an extent that the mid-brain forms the termination of the long axis of the body. At a later period in development the cerebral hemispheres come to be placed at the front end of the head; but the original nick or bend of the floor of the brain is never got rid of. It is obvious that in dealing with the light thrown by embryology on the ancestral form of the Chordata the significance of this peculiar character of the head of many vertebrate embryos must be discussed. Is the constancy of this character to be explained by supposing that at one period vertebrate ancestors had a head with the same features as the embryonic head of existing Vertebrata? This is the most obvious explanation, but it does not at the same time appear to me satisfactory. In the first place the mouth is so situated at the time of the maximum cranial flexure that it could hardly have been functional; so that it is almost impossible to believe that an animal with a head such as that of these embryos can have existed.