54 



THE HYPOBLAST. 



The posterior part of the alimentary canal retains for a 

 longer time its primitive condition. Finally however it also 

 becomes closed in, by the lips of the blastopore at the hind end 

 of the embryo meeting and uniting. The peculiarity of the 

 closing in of the posterior part of the alimentary canal consists 

 in the fact that a similar continuity to that in Amphioxus 

 obtains between the neural and alimentary canals. This is due 

 to the medullary folds being continuous at the end of the tail 

 with the lips of the blastopore, which close in the hind end of 

 the alimentary canal ; so that, when the 

 medullary folds unite to form a canal, this 

 canal becomes continuous with the ali- 

 mentary canal, which is closed in at the 

 same time. In other words, the medullary 

 folds assist in enveloping the blastopore 

 which does not therefore become absolutely 

 closed, but opens into the floor of the 

 neural canal. It will afterwards be shewn 

 that it is only the posterior part of the 

 blastopore that becomes closed during the 

 above process, and that the anterior and 

 ventral part long remains open. The general 

 arrangement of the parts, at the time when 

 the hind end of the mesenteron is first 

 closed, is shewn in fig. 27. The same 

 points may be seen in the diagrammatic 

 longitudinal section fig. 19 C. 



The middle portion of the alimentary 

 tract is the last to be closed in since it 

 remains till late in embryonic life as the 

 umbilical or vitelline canal, connecting the 

 yolk-sack with the alimentary cavity. The 

 umbilical canal falls into the alimentary 

 tract immediately behind the entrance of 

 the hepatic duct. 



At a fairly early stage of development a rod is constricted off 

 from the dorsal wall of the alimentary canal (figs. 27* and 23 x], 

 which is known as the subnotochordal rod. It is placed immedi- 

 ately below the notochord, and disappears during embryonic life. 



FIG. 27*. TRANSVERSE 

 SECTION THROUGH THE 

 TAIL REGION OF A PRIS- 

 TIURUS EMBRYO OF THE 

 SAME AGE AS FIG. 28 E. 



df. dorsal fin ; s/>.c. 

 spinal cord ; pp. body 

 cavity ; sf. splanchnic 

 layer of mesoblast ; so. 

 somatic layer of meso- 

 blast ; mp. commencing 

 differentiation of mus- 

 cles; ch. notochord; x. 

 sub-notochordal rod aris- 

 ing as an outgrowth of 

 the dorsal wall of the 

 alimentary tract ; al. ali- 

 mentary tract. 



