5a shews the formation of the thin roof of the fourth ventricle by a divarication of the two lateral halves of the brain.
Both sections shew the commencing formation of the thyroid body (th) at the base of the mandibular arch.
They also illustrate the formation of the visceral clefts by an outgrowth from the alimentary tract without any corresponding ingrowth of the external epiblast.
Fig. 6. Section through the hind-brain of a somewhat older Torpedo embryo. Zeiss A, ocul. 1.
The section shews (1) the attachment of a branch of the vagus to the walls of the hind-brain. (2) The peculiar form of the hind-brain.
Fig. 7. Transverse section through the head of a Pristiurus embryo belonging to a stage intermediate between I and K, passing through both the fore-brain and the hind-brain. Zeiss A, ocul. 1.
The section illustrates (1) the formation of the pituitary body (pt) from the mouth involution (m), and proves that, although the wall of the throat (al) is in contact with the mouth involution, there is by this stage no communication between the two. (2) The eye. (3) The sections of the body-cavity in the head (1pp, 2pp). (4) The fifth nerve (v.) and the seventh nerve (vii).
Fig. 8. Transverse section through the brain of a rather older embryo than fig. 7. It shews the ventral junction of the anterior sections of the body-cavity in the head (1pp).
Figs. 9a and 9b. Two longitudinal sections through the brain of a Pristiurus embryo belonging to a stage intermediate between I and K. Zeiss A, ocul. 1.
9a is taken through the median line, but is reconstructed from two sections. It shews (1) The divisions of the brain—The cerebrum and thalamencephalon in the fore-brain; the mid-brain; the commencing cerebellum in the hind-brain. (2) The relation of the mouth involution to the infundibulum. (3) The termination of the notochord.