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.
From the hinder border of the auditory region to the end of the branchial region the mesoblast is dorsally divided into myotomes, which nearly, though not quite, correspond in number with the branchial pouches.
The growth of the myotomes would seem, as might be anticipated from their independent innervation, not to be related to that of the branchial pouches, so that there is a want of correspondence between these parts, the extent of which varies at different periods of life. The relation between the two in an old larva is shewn in [fig. 47].
Fig. 46. Eye of a larva of Petromyzon nine days after hatching.
l. lens; r. retina.
The section passes through one side of the lens.
The head of the larva of Petromyzon differs very strikingly in general appearance from that of the normal Vertebrata. This is at once shewn by a comparison of [fig. 43] with [fig. 29]. The most important difference between the two is due to the absence of a pronounced cranial flexure in Petromyzon; an absence which is in its turn probably caused by the small development of the fore-brain.