162 A. SEDaWICK. 



into a canal open at either end. The neural canal therefore 

 owed its origin to the requirements of protection and respira- 

 tion. When once formed it must have continued to discharge 

 some function, otherwise it would have atrophied, as we know is 

 the habit of useless structures. The function of the canal at this 

 stage of its evolution is the subject of the present discussion. 



The relations of the neural canal at the siphon stage, which 

 is well marked in the development of Ascidians and Amphioxus, 

 are well known to all students of embryology. It is open 

 behind into the hind end of the alimentary canal and in front, 

 in the cephalic region of the body. 



How it acquired its opening into the alimentary canal is 

 perhaps hard to understand. The discussion of this question 

 involves the discussion of a still more difficult question, viz. the 

 relation of the permanent anus to the blastopore ; this I reserve 

 for a subsequent occasion.^ I may, however, point out here 

 that 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. Weldou^ 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. 



' Vide self, "On the Origin of Metameric Segmentation, &c.," 'Quart. 

 Journ. Micr. Rci.,' and tliese " Studies." 



• ' Quart. Journ. of Microscopical Science,' 1883; and these ''Studies," 

 pp. 5, 6, 



