Fig. 252. Longitudinal section through the brain of a young Pristiurus embryo.
cer. commencement of cerebral hemisphere; pn. pineal gland; In. infundibulum; pt. ingrowth of mouth to form the pituitary body; mb. mid-brain; cb. cerebellum; ch. notochord; al. alimentary tract; Iaa. artery of mandibular arch.
The optic vesicles become gradually constricted off from the fore-brain in a direction obliquely backwards and downwards. They remain, however, attached to it at the anterior extremity of the base of the fore-brain ([fig. 251], op.v.). While the above changes are taking place in the optic vesicles the anterior part of the fore-brain becomes prolonged, and at the same time somewhat dilated. At first there is no sharp boundary between the primitive fore-brain and its anterior prolongation, but there shortly appears a constriction which passes from above obliquely forwards and downwards. This constriction is shallow at first, but soon becomes much deeper, leaving however the cavities of the two divisions of the fore-brain united ventrally by a somewhat wide canal ([fig. 252]).
Of these two divisions the posterior becomes the thalamencephalon, while the anterior and larger division (cer) forms the rudiment of the cerebral hemispheres and olfactory lobes. For a considerable period this rudiment remains perfectly simple, and exhibits no signs, either externally or internally, of a longitudinal constriction dividing it into two lobes.
From the above description it may be concluded that the rudiment of the cerebral hemispheres is contained in the original fore-brain. In spite however of their great importance in all the Craniata, it is probable that the hemispheres were either not present as distinct structures, or only imperfectly separated from the thalamencephalon, in the primitive vertebrate stock.
The thalamencephalon. The thalamencephalon varies so slightly in structure throughout the Vertebrate series that a general description will suffice for all the types.
It forms at first a simple vesicle, the walls of which are of a nearly uniform thickness and formed of the usual spindle-shaped cells.
Fig. 253. Diagrammatic vertical section through the head of a larva of Petromyzon.
The larva had been hatched three days, and was 4.8 mm. in length. The optic and auditory vesicles are supposed to be seen through the tissues.
c.h. cerebral hemisphere; th. optic thalamus; in. infundibulum; pn. pineal gland; mb. mid-brain; cb. cerebellum; md. medulla oblongata; au.v. auditory vesicle; op. optic vesicle; ol. olfactory pit; m. mouth; br.c. branchial pouches; th. thyroid involution; v.ao. ventral aorta; ht. ventricle of heart; ch. notochord.
The cavity it contains is known as the third ventricle. Anteriorly it opens widely into the cerebral rudiment, and posteriorly into the ventricle of the mid-brain. The opening into the cerebral rudiment becomes the foramen of Munro.