The next change of any importance is the commencement of the formation of the head. The region of the head first becomes distinguishable by the flattening out of the germ at its front end. The flattened-out portion of the germ grows rapidly, and forms a spatula-like termination to the embryo (Pl. 3, fig. 8). In the region of the head the medullary groove is at first totally absent (vide section, Pl. 3, fig. 8_a_). Indeed, as can be seen from fig. 8_b_, the laminæ dorsales, so far from bending up at this stage, actually bend down in the opposite direction. I am at present quite unable even to form a guess what this peculiar feature of the brain means. It, no doubt, has some meaning in reference to the vertebrate ancestry if we could only discover it. The peculiar spatula-like flattened condition of the head is also (vide _loc. ant. cit._) apparently found in the Sturgeons; it must therefore almost undoubtedly be looked upon as not merely an accidental peculiarity. While these changes have been taking place in the head not less important changes have occurred in the remainder of the body. In the first place the two caudal lobes have increased in size, and have become, as it were, pushed in together, leaving a groove between them (fig. 8, _ts_). They are very conspicuous objects, and, together with the spatula-like head, give the whole embryo an almost comical appearance. The medullary canal has by this time become completely closed in the region of the tail (figs. 8 and 8_b_). It is still widely open in the region of the back, and, though more nearly closed again in the neck, is, as I have said, flattened out to nothing in the head. The groove[15] between the two caudal lobes must not be confused (as may easily be done) with the medullary groove, which by the time the former groove has become conspicuous is a completely closed canal. Footnote 15: This groove is the only structure which it seems possible to compare with the so-called "primitive groove" of Birds. It is, however, doubtful whether they are really homologous. The vertebral plates are not divided (vide fig. 7) into a somatopleuric and splanchnopleuric layer by this stage, except in the region of the head (vide fig. 8_b_, _pp_), where there is a distinct space between the two layers, which is undoubtedly homologous with the pleuro-peritoneal cavity of the hinder portion of the body. It is probably the same cavity which Oellacher (_loc. cit._) calls in Osseous fishes the pericardial cavity. In the Dog-fish, at least, it has no connection with the pericardium. Of its subsequent history I shall say a few words when I come to speak of the later stages.