In a still anterior section (fig. 3_a_) no cavity is any longer present in the mesoblast; whilst in sections taken from the foremost part of the head no mesoblast is to be seen (vide Pl. 10, fig. 5, taken from the front part of the head of the embryo represented in Pl. 8, fig. F). A continuation of the body-cavity into the head has already been described by Oellacher[182] for the Trout: but he believes that the cavity in this part is solely related to the formation of the pericardial space. Footnote 182: _Zeitschrift f. wiss. Zoologie_, 1873. The condition of the mesoblast undergoes no important change till the end of the period treated of in this chapter. The masses of mesoblast which form the tail-swellings become more conspicuous (Pl. 10, fig. 4); and indeed their convexity is so great that the space between them has the appearance of a median groove, even after the closure of the neural canal in the caudal region. In embryos of stage G, which may be considered to belong to the close of this period, eighteen protovertebræ are present both in Pristiurus and Torpedo embryos. _The Alimentary Canal._ The alimentary canal at the commencement of this period (stage B) forms a space between the embryo and the yolk, ending blindly in front, but opening posteriorly by a widish slit-like aperture, which corresponds to the anus of Rusconi (Pl. 7, fig. 7). The cavity anteriorly has a more or less definite form, having lateral walls, as well as a roof and floor (Pl. 7, figs. 10_b_ and 10_c_). Posteriorly it is not nearly so definitely enclosed (Pl. 7, fig. 10_a_). The ventral wall of the cavity is formed by yolk. But even in stage B there are beginnings of a cellular ventral wall derived from an ingrowth of cells from the two sides.