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 alimentary 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. [FIG. 27*. TRANSVERSE SECTION THROUGH THE TAIL REGION OF A PRISTIURUS EMBRYO OF THE SAME AGE AS FIG. 28 E. _df_. dorsal fin; _sp.c_. spinal cord; _pp_. body cavity; _sp_. splanchnic layer of mesoblast; so. somatic layer of mesoblast; _mp_. commencing differentiation of muscles; _ch_. notochord; _x_. subnotochordal rod arising as an outgrowth of the dorsal wall of the alimentary tract; _al_. alimentary tract.] 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 immediately below the notochord, and disappears during embryonic life. _General features of the Elasmobranch embryo at successive stages._