498 EMBEYOLOGY OF THE LOWEK VEETEBEATES en. 



regarded as being continued forwards by a potential slit, represented 

 e.g. by the primitive streak of other forms, it will be realized how 

 close is the resemblance to the conditions in Peripatus. The pre-anal 

 portions of the neural rudiment in Lepidosiren come together in the 

 mesial plane to form the spinal cord, while the postanal portion 

 flattens out and disappears so that the anal opening comes to lie 

 entirely behind the posterior limit of the central nervous system. It 

 is clear that if the development of the anal opening were delayed 

 until the neural folds had already come together it would make its 

 appearance completely behind the central nervous rudiment and with 

 no obvious connexion with it. This is very possibly the case in 

 Vertebrates other than those mentioned. 



Although the anal opening of Vertebrates is thus brought into 

 the relations with the nervous system that we shauld expect on the 

 protostoma hypothesis there is no such definite evidence in the 

 case of the mouth. It is true that in some cases the dorsal 

 furrow has been traced to the neighbourhood of the mouth and 

 that the mouth opening has in some cases at first the form of a 

 sagittally placed slit, but in no case, up to the present, has the 

 neural rudiment been traced round in front of the mouth. This 

 difficulty however is greatly lessened when we correlate the facts just 

 mentioned in regard to the anal opening in Lepidosiren with the 

 relatively late appearance of the mouth opening of Vertebrates as 

 discussed on p. 193. It may well be that the non-inclusion of the 

 mouth opening within the obvious neural rudiment is due simply to 

 the pre-oral parts of the medullary folds having already flattened out 

 and disappeared before the oral opening makes its appearance. If 

 this is the case it carries with it the interesting consequence that the 

 supra-oesophageal or pre-oral ganglia of Peripatus have disappeared in 

 the Vertebrate and it is therefore waste of energy to discuss what 

 parts of the brain of a Vertebrate are homologous with the supra- 

 oesophageal ganglia of Invertebrates. 



This Protostoma idea, dealing as it does with extremely remote 

 phases of the Vertebrate phylogeny, must not be looked on as a 

 definitely proved theory, nor can it be expected ever to reach that 

 dignity, but it is a fascinating working hypothesis which serves, and 

 which alone serves, to link together and in a sense explain a 

 considerable body of otherwise mysterious and apparently inexplicable 

 facts of Vertebrate embryology. 1 



(6) The Vertebrate .Head. — The two phyla of the animal, 

 kingdom which have reached the highest stage of evolutionary 

 development — the Arthropoda and the Vertebrata — are alike char- 

 acterized by the possession of a well -developed head. In the 



1 In considering the difficulties in the way of the theory afforded by cases where 

 the gastrula becomes roofed in by a process of simple backgrowth without any trace 

 of protostoma (e.g. Amphioxus), it is well to bear in mind the parallel case of the 

 amnion — of which a large portion may be formed by simple backgrowth, although 

 the sero-anmiotic isthmus and the ingrowth of mesoderm from the two sides seem to 

 point clearly to a former formation by the meeting of two lateral folds. 



