292 



EMBRYOLOGY. 



7JC 



the posterior end of the body. Whereas the oral sinus comes in 

 contact with the anterior end of the fore gut, the formation of the 

 anus does not take place at the posterior end of the embryonic intes- 

 tine, which is occupied by the primitive mouth [blastopore], but at 

 some distance in front of it. (Compare also fig. 126, that of the 

 Chick, in which the region where the anal pit is to be formed is 

 designated by the letters an.) Consequently in the embryos of 

 Vertebrates, when the anus has broken through, the embryonic in- 

 testinal tube is still continued for some distance back of the anus to 

 the primitive mouth. This portion is designated as the post- anal or 

 caudal gut (fig. 126 p.a.g.). The latter designation is appropriate, 

 because the part of the body which lies behind the inus, in which is 



enclosed the part 

 of the intestine 

 under considera- 

 tion, becomes the 

 tail-end of the 

 embryo. 



The post-anal 

 gut appears to be 

 established as a 

 shorter or longer 

 tract in all Ver- 

 tebrates ; it has 

 already been ob- 

 served in the most 



widely different animals by several investigators : first by KOWALEVSKY 

 in Amphioxus, the Acipenseridse, Selachians, and Teleosts ; then 



by GOETTE, BOBRETZKY, BALFOUR, HlS, KoLLIKER, GASSER, BRAUN, 



BONNET, and others in the Amphibia, Selachians, Birds (fig. 126 

 p.a.g.), and Mammals. In the Selachians (Scyllium) the post-anal 

 section at the time of its greatest development attains about one- 

 third the length of the whole alimentary canal. It exhibits at its 

 end a small vesicular enlargement, which communicates with the 

 neural tube by means of a narrow opening. In an advanced embryo 

 of Bombinator it is also to be seen well developed, as shown in the 

 sagittal section fig. 159. It begins at the place marked by an, 

 at which the epidermis has sunk down to form the anal pit (an) and 

 at which it has united with the intestine, immediately behind the 

 mass of yolk-cells collected in the ventral wall of the latter. From 

 this point it runs backward as a narrow but open tube, and bends 



Fig. 159. Sagittal section through an advanced embryo of 



Bombinator, after GOETTE. 

 m, Mouth ; an, anus ; I, liver ; ne, nexarenteric canal ; me, medullary 



tube ; ch, chorda ; pn, pineal gland. 



