According to Scott (No. [87]) a hyomandibular pouch forming the eighth pouch is formed in front of the pouch already defined as the hyobranchial. It disappears early and does not acquire gill folds[28]. The tissue forming the line of insertion of the velum appears to me to represent the mandibular arch. The grounds for this view are the following:
(1) The structure in question has exactly the position usually occupied by the mandibular arch.
(2) There is present in late larvæ (about 20 days after hatching) an arterial vessel, continued from the ventral prolongation of the bulbus arteriosus along the insertion of the velum towards the dorsal aorta, which has the relations of a true branchial artery.
On the ventral aspect of the branchial region is placed a sack ([figs. 42], h, and [43], th), which extends from the front end of the branchial region to the fourth cleft. At first it constitutes a groove opening into the throat above ([fig. 44]), but soon the opening becomes narrowed to a pore placed between the second and third of the permanent branchial pouches ([fig. 43], th). In Ammocœtes[29] the simple tube becomes divided, and assumes a very complicated form, though still retaining its opening into the branchial region of the throat. In the adult it forms a glandular mass underneath the branchial region of the throat equivalent to the thyroid gland of higher Vertebrates.
On the ventral aspect of the head, and immediately in front of the mouth, is placed the olfactory pit ([fig. 43], ol). It is from the first unpaired, and in just-hatched larvæ simply forms a shallow groove of thickened epiblast at the base of the front of the brain. By the stage represented in [fig. 43] the ventral part of the original groove is prolonged into a pit, extending backwards beneath the brain nearly up to the infundibulum.
Fig. 44. Diagrammatic transverse sections through the branchial region of a young larva of Petromyzon. (From Gegenbaur; after Calberla.)
d. branchial region of throat.
On the side of the head, nearly on a level with the front end of the notochord, is placed the eye ([fig. 43], op). It is constituted ([figs. 45] and [46]) of a very shallow optic cup with a thick outer (retinal) layer, and a thin inner choroid layer. In contact with the retinal layer is placed the lens. The latter is formed as an invagination of the skin; to which it is still attached in the just-hatched larva ([fig. 45]). The eye only differs at this stage from that of other Vertebrata in its extraordinarily small size, and the rudimentary character of its constituent parts.
The auditory sack is a large vesicle ([fig. 43], au.v.), placed at the side of the brain opposite the first persistent branchial pouch.
The brain is formed of the usual vertebrate parts[30], but is characterized by the very slight cranial flexure. The fore-brain consists ([fig. 43]) of a thalamencephalon (th) and an undivided cerebral rudiment (ch). To the roof of the thalamencephalon is attached a flattened sack (pn) which is probably the pineal gland. The floor is prolonged into an infundibulum (in) which contains a prolongation of the third ventricle. The lateral walls of the cerebral rudiment are much thickened.
Behind the thalamencephalon follows the mid-brain (mb), the sides of which form the optic lobes, and behind this again the hind-brain (md); the front border of the roof of which is thickened to form the cerebellum (cb). The medulla passes without any marked line of demarcation into the spinal cord.