35S p. M. BALFOUR. 



(c) The lateral plate. From the somatic layer of this is 

 derived the connective tissue of the ventral half of the body ; 

 the mesoblast of the limbs, including probably the muscles, 

 and certainly the skeleton. From its splanchnic layer are 

 derived the muscles and connective tissue of the alimentary 

 canal. 



(3) The spinal nerves are developed independently of the 

 protovertebrse, so that the protovertebrse of the Elasmo- 

 branchii do not appear to be of such a complicated structure 

 as the protovertebrse of Birds. 



The Digestive Canal. 



I do not intend to enter into the whole history of the 

 digestive canal, but to confine myself to one or two points 

 of interest connected with it. These fall under two heads : 



(1) The history of the portion of the digestive canal 

 between the anus and the end of the tail where the digestive 

 canal opens into the neural canal. 



(3) Certain less well-known organs derived from the 

 digestive canal. 



The anus is a rather late formation, but its position 

 becomes very early marked out by the hypoblast of the 

 digestive canal approaching at that point close to the surface, 

 whilst receding to some little distance from it on either side. 

 The portion of the digestive tract I propose at present dealing 

 with is that between this point, which I will call, for the sake 

 of brevity, the anus, and the hind end of the body. This 

 portion of the canal is at first very short ; it is elliptical in 

 section, and of rather a larger bore than the remainder of 

 the canal. Its diameter becomes, however, slightly less as it 

 approaches the tail, dilating again somewhat at its extreme 

 end. It is lined by a markedly columnar epithelium. 

 Though at first very short, its length increases with the 

 growth of the tail, but at the same time its calibre con- 

 tinually becomes smaller as compared with the remainder of 

 the alimentary canal. 



It commences to become smaller, first of all, near, though 

 not quite, at its extreme hind end, and thus becomes of a 

 conical shape; the base of the cone being just behind the 

 anus, while the apex of the cone is situated a short distance 

 from the hind end of the embryo. The extreme hind end, 

 however, at the same time does not diminish in size, and 

 becomes relatively (if not also absolutely) much larger in 

 diameter than it was at first, as compared with the remainder 

 of the digestive canal. It becomes, in fact, a vesicle or 

 vesicular dilatation at the end of a conical canal. 



