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. [30] Max Schultze's statements as to the structure and histology of the brain are very inadequate in the present state of our knowledge. 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. [FIG. 45. HORIZONTAL SECTION THROUGH THE HEAD OF A JUST-HATCHED LARVA OF PETROMYZON SHEWING THE DEVELOPMENT OF THE LENS OF THE EYE. _th.c._ thalamencephalon; _op.v._ optic vesicle; _l._ lens of eye; _h.c._ head cavity.] The histological differentiation of the brain has already proceeded to some extent; and it has in the main the same character as the spinal cord. Before the larva has been hatched very long a lateral investment of white matter is present throughout. The notochord (_ch_) is continued forwards in the head to the hinder border of the infundibulum. It is slightly flexed anteriorly.