Great changes have been effected in the head, resulting in a diminution of the cranial flexure. This diminution is nevertheless apparent rather than real, and is chiefly due to the rapid growth of the rudiment of the cerebral hemispheres. The three main divisions of the brain may still be clearly seen from the surface. Posteriorly is situated the hind-brain, now consisting of the medulla oblongata and cerebellum. At the anterior part of the medulla is to be seen the thin roof of the fourth ventricle, and anteriorly to this again the roof becomes thickened to form the rudiment of the cerebellum. In front of the hind-brain lies the mid-brain, the roof of which is formed by the optic lobes, which are still situated at the front end of the long axis of the embryo.
Beyond the mid-brain is placed the fore-brain, whose growth is rapidly rendering the cranial flexure imperceptible.
The rudiments of the nasal sacs are now clearly visible as a pair of small pits. The pits are widely open to the exterior, and are situated one on each side, near the front end of the cerebral hemispheres. Five visceral clefts are open to the exterior, and in them the external gills have commenced to appear (L´).
The first cleft is no longer similar to the rest, but has commenced to be metamorphosed into the spiracle.
Accompanying the change in position of the first cleft, the mandibular arch has begun to bend round and enclose the front as well as the side of the mouth. By this change in the mandibular arch the mouth becomes narrowed in an antero-posterior direction.
M.
Of this embryo the head alone has been represented. Two views of it are given, one (M) from the side and the other (M´) from the under surface. The growth of the front part of the head has considerably diminished the prominence of the cranial flexure. The full complement of visceral clefts is now present—six in all. But the first has already atrophied considerably, and may easily be recognized as the spiracle. In Scyllium, there are present at no period more than six visceral clefts. The first visceral arch on each side has become bent still further round, to form the front border of the mouth. The opening of the mouth has in consequence become still more narrowed in an antero-posterior direction. The width of the mouth in this direction, serves for the present and for some of the subsequent stages as a very convenient indication of age.
N.
The limbs, or paired fins, have now acquired the general features and form which they possess in the adult.
The unpaired fins have now also become divided in a manner not only characteristic of the Elasmobranchii but even of the genus Scyllium.