ON THE ANCESTRAL FORM OF THE CHORD AT A. 267 



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. 



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

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

 in the Ichthyopsida. In Amphibia, Teleostei, Ganoidse and Petromyzontidse, 

 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 jihase 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 Elasmo- 

 branchii, 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 

 embryonic 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 

 development. The flexure at the base of the brain appears however to be a 

 typical 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 sup- 

 posing 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 selectionin bring- 

 ing about this form of 

 head during foetal life. 



Post-anal gut and 

 nenrenteric canal. 



One of the most re- 

 markable structures in 

 the trunk is the post- 

 anal gut (fig. 197). Its 

 structure is fully dealt 

 with in the chapter on 

 the alimentary tract,but 

 attention may here be 

 called to the light which Fig. 197. Longitudinal section through an ad- 



it appears to throw on ^''^''^'^ =^^^^« °^ Bombinatoe. (After Gotte.) 

 +V>£i /,V.r.».r,o+^>.c^f +1 ^ '"• mouth; an. anus; I. liver; ne. neurenterio 



tne cnaractersot tnean- ^^^^y^ ^ medullary canal; ch. notochord; ijn. pineal 

 cestor 01 the Chordata. gland. 



P»^ 



