1883.] of the central nervous system of Vertebrata. 327 



development points to the fact that the blastopore was placed 

 within the medullary plate, and that therefore on the conversion 

 of the medullary groove into a canal, the alimentary canal would 

 open into the hind end of that canal, and the two tubes would 

 open to the exterior together. 



It is quite clear that the anus of existing vertebrata is not in 

 the position of the primitive anus or blastopore of ancestral forms, 

 and it has been commonly supposed that the present anus is a 

 new formation. That the blastopore closes is certain, but it has 

 been recently pointed out by Mr Weldon 1 that the present anus 

 occurs along the line of the blastopore : and I hope soon to be 

 able to show that the permanent anus is identical with the 

 blastopore, the temporary closure of which is simply a matter 

 of developmental necessity. However this may be, there can 

 be but little doubt that this relation of the hind end of the neural 

 canal to the alimentary canal has existed in the ancestors of 

 vertebrates. 



To return to the main question, What is the function of 

 the neural canal at this stage ? It seems to me that that function 

 must have been in the main a respiratory one. The water entered 

 the canal by the anterior pore, was driven through it by the cilia, 

 and at the hind end passed through the neurenteric canal into the 

 alimentary canal, and so out by the anus. In support of this I 

 appeal to certain well-known physiological and anatomical facts. 



In the first place, in the vertebrata the brain requires more 

 oxygen for its well being than any other tissue of the body, and in 

 those vertebrates, e.g. Amphibia and iSauropsida, in which there is 

 only one ventricle, special arrangements are present to ensure 

 a supply of pure arterial blood to the head. In the second place, 

 in the tracheate animals the central nervous system has a specially 

 rich supply of tracheae. Finally in Certain worms, e.g. Nemertines, 

 Aphrodite, the whole nervous system contains haemoglobin, which 

 may be supposed to exercise a special attraction for oxygen, and 

 hold it in a convenient state for the use of the nerve-cells. 



It is interesting to notice here that in most of the animals I 

 have just mentioned, in which there are special arrangements for 

 the respiration of the nervous system, the vascular system is but 

 little developed. It seems probable that the ancestral vertebrate 

 with the siphon stage of neural canal was without a well-developed 

 vascular system. When this and definite respiratory organs became 

 developed, a new stage in the evolution of the neural canal was 

 reached, in which it lost its respiratory function, this being 

 assumed by the vascular system. As a result of this, the anterior 

 and posterior opening became closed. This brings us to the 



' Quart. J. o/Mic. Science, 1883. 



