The sides of the thalamencephalon become very early thickened to form the optic thalami, which constitute the most important section of the thalamencephalon. They are separated, in Mammalia at all events, on their inner aspect from the infundibular region by a somewhat S-shaped groove, known as the sulcus of Munro, which ends in the foramen of Munro. They also become in Mammalia secondarily united by a transverse commissure, the grey or middle commissure, which passes across the cavity of the third ventricle. This commissure is probably homologous with, and derived from, a commissural band in the roof of the thalamencephalon, placed immediately in front of the pineal gland which is well developed in Elasmobranchii ([fig. 254]).
The roof undergoes more complicated changes. It becomes divided, on the appearance of the pineal gland as a small papilliform outgrowth (the development of which is dealt with separately), into two regions—a longer anterior in front of the pineal gland and a shorter posterior. The anterior region becomes at an early period excessively thin, and at a later period, when the roof of the thalamencephalon is shortened by the approach of the cerebral hemispheres to the mid-brain, it becomes (vide [figs. 250] and [255], chd 3, and [254]) considerably folded, while at the same time a vascular plexus is formed in the pia mater above it. On the accomplishment of these changes it is known as the tela choroidea of the third ventricle.
In the roof of the third ventricle behind the pineal gland there appear in Elasmobranchii, the Sauropsida and Mammalia transverse commissural fibres, forming a structure known as the posterior commissure, which connects together the two optic thalami.
The most remarkable organ in the roof of the thalamencephalon is the pineal gland, which is developed in most Vertebrates as a simple papilliform outgrowth of the roof, and is at first composed of cells similar to those of the other parts of the central nervous system ([figs. 250], [252], [254] and [255], pn or pin). In the lower Vertebrata it is directed forwards, but in Mammalia, and to some extent in Aves, it is directed backwards.
In Amphibia it is described by Götte (No. [296]) as being a product of the point where the roof of the brain remains latest attached to the external skin.
The figure which Götte gives to prove this does not appear to me fully to bear out his conclusion; which if true is very important. Although I directed my attention specially to this point, I could find no indication in Elasmobranchii of a process similar to that described by Götte, and his observations have not as yet been confirmed for other Vertebrates. Götte compares the pineal gland to the long-persisting pore which leads into the cavity of the brain in the embryo of Amphioxus, and we might add the Ascidians, and, should his facts be confirmed, the conclusion he draws from them would appear to be well founded.
The later stages in the development of the pineal gland in different Vertebrates have not in all cases been fully worked out[165].
Fig. 255. Longitudinal vertical section through the anterior part of the brain of an embryo rabbit of four centimetres. (After Mihalkovics.)
The section passes through the median line so that the cerebral hemispheres are not cut; their position is however indicated in outline.
spt. septum lucidum formed by the coalescence of the inner walls of part of the cerebral hemispheres; cna. anterior commissure; frx. vertical pillars of the fornix; cal. genu of corpus callosum; trm. lamina terminalis; hms. cerebral hemispheres; olf. olfactory lobes; acl. artery of corpus callosum; fmr. position of foramen of Munro; chd3. choroid plexus of third ventricle; pin. pineal gland; cmp. posterior commissure; bgm. lamina uniting the lobes of the mid-brain; chm. optic chiasma; hph. pituitary body; inf. infundibulum; pns. pons Varolii; pde. cerebral peduncles; agd. iter.