322 POST-ANAL GUT. 



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 Elasmobran- 

 chii. The peculiarity in question, which is characteristically shewn in fig. 

 196, consists in the cerebral hemispheres and thalamencephalon being ven- 

 trally 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 ori- 

 ginal 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 func- 

 tional ; so that it is almost impossible to believe that an animal with a 

 head such as that of these embryos can have existed. 



Then again, this type of embryonic head is especially characteristic of 

 the Amniota, all of which are developed in the egg. It is not generally so 

 marked in the Ichthyopsida. In Amphibia, Teleostei, Ganoidae and Petromy- 

 zontidte, the head never completely acquires the peculiar characteristic form 

 of the head of the Amniota, and all these forms are hatched at a relatively 

 much earlier phase of development, so that they are leading a free existence 

 at a stage when the embryos of the Amniota are not yet hatched. The only 

 Ichthyopsidan type with a head like that of the Amniota is the Elasmobran- 

 chii, and the Elasmobranchii are the only Ichthyopsida which undergo the 

 major part of their development within the egg. 



These considerations appear to shew that the peculiar characters of the 

 embryonic head above alluded to are in some way connected with an em- 

 bryonic as opposed to a larval development ; and for reasons which are 

 explained in the section on larval forms, it is probable that a larval develop- 

 ment is a more faithful record of ancestral history than an embryonic deve- 

 lopment. The flexure at the base of the brain appears however to be a typi- 

 cal vertebrate character, but this flexure never led to a conformation of the 

 head in the adult state similar to that of the embryos of the Amniota. The 

 form of the head in these embryos is probably to be explained by supposing 

 that some advantage is gained by a relatively early development of the brain, 

 which appears to be its proximate cause ; and since these embryos had not 

 to lead a free existence (for which such a form of the head would have been 

 unsuited) there was nothing to interfere with the action of natural selection 

 in bringing about this form of head during foetal life. 



Post-anal gut and neurenteric canal. One of the most 



