It is not absolutely clear whether the notochord is to be regarded as an axial differentiation of the hypoblast, or as an axial differentiation of the lower layer cells. The facts of development both in Amphioxus and Elasmobranchii tend towards the former view; but the nearly simultaneous differentiation of the notochord and the mesoblastic plates lends some support to the supposition that the notochord may be merely a median plate of mesoblast developed slightly later than the two lateral plates. The alimentary canal or mesenteron was left as a space between the hypoblast and the yolk, ending blindly in front, but opening behind by a widish aperture, the blastopore or anus of Rusconi (_vide_ fig. 19 B). [FIG. 26. SECTION THROUGH THE ANTERIOR PART OF A PRISTIURUS EMBRYO TO SHEW THE FORMATION OF THE ALIMENTARY TRACT. _Ch._ notochord; _hy._ hypoblast; _al._ alimentary tract; _na._ cells passing in from the yolk to form the ventral wall of the alimentary tract.] The conversion of this irregular cavity into a closed canal commences first of all at the anterior extremity. In this conversion two distinct processes are concerned. One of these is a process of folding off of the embryo from the blastoderm. The other is a simple growth of cells independent of any fold. To the first of these processes the depth and narrowness of the alimentary cavity is due; the second is concerned in forming its ventral wall. The process of the folding off of the embryo from the blastoderm resembles exactly the similar process in the embryo bird. The fold is a perfectly continuous one round the front end of the embryo, but may be conveniently spoken of as composed of a head-fold and two lateral folds. Of far greater interest than the nature of these folds is the formation of the ventral wall of the alimentary canal. This originates in a growth of cells from the two sides to the middle line (fig. 26). The cells for it are not however mainly derived from pre-existing hypoblast cells, but are formed _de novo_ around the nuclei of the yolk which have already been spoken of (fig. 26, _na_). The ventral wall of the mesenteron is in fact, to a large extent at any rate, formed as a differentiation of the primitive yolk floor. [FIG. 27. LONGITUDINAL VERTICAL SECTION OF AN EMBRYO SLIGHTLY YOUNGER THAN THAT IN FIG. 26 D. The section shews the communication which exists between the neural and alimentary canals. _nc._ neural canal; _al._ alimentary tract; _Ch._ notochord; _Ts._ tail swelling.] The folding off and closing of the alimentary canal in the anterior part of the body proceeds rapidly, and not only is a considerable tract of the alimentary canal formed, but a great part of the head is completely folded off from the yolk before the medullary groove is closed.