The alimentary canal in the Chordata is always formed of three sections, analogous to those so universally present in the Invertebrata. These sections are (1) the mesenteron lined by hypoblast; (2) the stomodæum or mouth lined by epiblast, and (3) the proctodæum or anal section lined like the stomodæum by epiblast.
Mesenteron.
The early development of the epithelial wall of the mesenteron has already been described (Chapter XI.). It forms at first a simple hypoblastic tube extending from near the front end of the body, where it terminates blindly, to the hinder extremity where it is united with the neural tube by the neurenteric canal ([fig. 420], ne). It often remains for a long time widely open in the middle towards the yolk-sack.
It has already been shewn that from the dorsal wall of the mesenteron the notochord is separated off nearly at the same time as the lateral plates of mesoblast (pp. 292-300).
The subnotochordal rod. At a period slightly subsequent to the formation of the notochord, and before any important differentiations in the mesenteron have become apparent, a remarkable rod-like body, which was first discovered by Götte, becomes split off from the dorsal wall of the alimentary tract in all the Ichthyopsida. This body, which has a purely provisional existence, is known as the subnotochordal rod.
It develops in Elasmobranch embryos in two sections, one situated in the head, and the other in the trunk.
The section in the trunk is the first to appear. The wall of the alimentary canal becomes thickened along the median dorsal line ([fig. 412], x), or else produced into a ridge into which there penetrates a narrow prolongation of the lumen of the alimentary canal. In either case the cells at the extreme summit become gradually constricted off as a rod, which lies immediately dorsal to the alimentary tract, and ventral to the notochord ([fig. 413], x).
Fig. 412. 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´. portion of splanchnic mesoblast commencing to be differentiated into muscles; ch. notochord; x. subnotochordal rod arising as an outgrowth of the dorsal wall of the alimentary tract; al. alimentary tract.