268 POST-ANAL GUT. 



In face of the facts which are known with reference to the post-anal 

 section of the alimentary tract, it can hardly be doubted that this 

 portion of the alimentary tract must have been at one time func- 

 tional. This seems to me to be shewn (1) by the constancy and 

 persistence of this obviously now functionless rudiment, (2) by its 

 greater development in the lower than in the higher forms, (3) by 

 its relation to the formation of the notochord and subnotochordal 

 rod. 



If the above position be admitted, it is not permissible to shirk 

 the conclusions which seem necessarily to follow, however great the 

 difficulties may be which are involved in their acceptance. These 

 conclusions have in part already been dealt with by Dohrn in his 

 suggestive tract (No. 250). In the first place the alimentary canal 

 must primitively have been continued to the end of the tail ; and if 

 so, it is hardly credible that the existing anus can have been the 

 original one. Although, therefore, it is far from easy, on the physio- 

 logical principles involved in the Darwinian theory, to understand 

 the formation of a new anus^; it is nevertheless necessary to believe 

 that the present vertebrate anus is a formation acquired within the 

 group of the Chordata, and not inherited from some older group. 

 This involves a series of further consequences. The opening of the 

 urinogenital ducts into the cloaca must also be secondary, and it is 

 probable that the segmental tubes were primitively continued along 

 the whole post-anal region of the vertebrate tail, opening into the 

 body-cavity which embryology proves to have been originally present 

 there. They are in fact continued in many existing forms for some 

 distance behind the present anus. If the present anus is secondary, 

 there must have been a primitive anus, which was probably situated 

 behind the post-anal vesicle ; and therefore in the region of the neu- 

 renteric canal. The neurenteric canal is, however, the remnant of the 

 blastopore {vide p. 230). It follows, therefore, that the vertebrate blasto- 

 pore is probably almost, if not exa,ctly identical in position loith the 

 primitive anus. This consideration may assist in explaining the re- 

 markable phenomenon of the existence of the neurenteric canal. The 

 attempt has already been made to shew that the central canal of the 

 nervous system is really a groove converted into a tube and lined by the 

 external epidermis. This tube (as may be concluded from embryo- 

 logical considerations) was probably at first open posteriorly, and no 

 doubt terminated at the primitive anus. On the closure of the 

 primitive anal opening, the terminal portions of the post-anal gut 

 and the neural tube, may conceivably have been so placed that both 

 of them opened into a common cavity, which previously had commu- 

 nication with the exterior by the anus. Such an arrangement would 

 necessarily result in the formation of a neurenteric canal. It seems 

 not impossible that a dilated vesicle, often present at the end of the 



1 Dohrn (No. 250, p. 25) gives an explanation of the origin of the new anus 

 which does not appear to me quite satisfactory. 



