The olfactory pits are now developed, and are placed near the front of the head.
A great advance has taken place in the development of the visceral clefts and arches. The oral region is bounded behind by a well-marked mandibular arch, which is separated by a shallow depression from a still more prominent hyoid arch ([fig. 65], hy). Between the hyoid and mandibular arches a double lamella of hypoblast, which represents the hyomandibular cleft, is continued from the throat to the external skin, but does not, at this stage at any rate, contain a lumen.
The hyoid arch is prolonged backwards into a considerable opercular fold, which to a great extent overshadows the branchial clefts behind. The hyobranchial cleft is widely open.
Behind the hyobranchial cleft are four pouches of the throat on each side, not yet open to the exterior. They are the rudiments of the four branchial clefts of the adult.
The trunk has the usual compressed piscine form, and there is a well-developed dorsal fin continuous round the end of the tail, with a ventral fin. There is no trace of the paired fins.
Fig. 65. Embryo of Lepidosteus shortly before hatching.
ol. olfactory pit; sd. suctorial disc; hy. hyoid arch.
The anterior and posterior portions of the alimentary tract are closed in, but the middle region is still open to the yolk. The circulation is now fully established, and the vessels present the usual vertebrate arrangement. There is a large subintestinal vein.
The first of Agassiz’ embryos was hatched about ten days after impregnation. The young fish on hatching immediately used its suctorial disc to attach itself to the sides of the vessel in which it was placed.