For convenience of description I shall divide it into three regions, viz. (1) the floor, (2) the sides, and (3) the roof.
The floor becomes divided into two parts, an anterior part, giving origin to the optic nerves, in which is formed the optic chiasma; and a posterior part, which becomes produced into an at first inconspicuous prominence—the rudiment of the infundibulum ([fig. 252], In). This comes in contact with an involution from the mouth, which gives rise to the pituitary body ([fig. 252], pt), the development of which will be dealt with separately.
In the later stages of development the infundibulum becomes gradually prolonged, and forms an elongated diverticulum of the third ventricle, the apex of which is in contact with the pituitary body ([figs. 252], [254], in, and [figs. 250] and [255], inf).
Along the sides of the infundibulum run the commissural fibres connecting the floor of the mid-brain with the cerebrum.
Fig. 254. Longitudinal section through the brain of Scyllium canicula at an advanced stage of development.
cer. cerebral hemisphere; pn. pineal gland; op. th. optic thalamus, connected with its fellow by a commissure (the middle commissure). In front of it is seen a fold of the roof of the fore-brain, which is the choroid plexus of the third ventricle; op. optic chiasma; pt. pituitary body; in. infundibulum; cb. cerebellum; au.v. passage leading from the auditory vesicle to the exterior; mel. medulla oblongata; c.in. internal carotid artery.
In its later stages the infundibular region presents considerable variations in the different vertebrate types. In Fishes it generally remains very large, and permanently forms a marked diverticulum of the floor of the thalamencephalon. In Elasmobranchii the distal end becomes divided into three lobes—a median and two lateral. The lateral lobes appear to become the sacci vasculosi of the adult.
In Teleostei peculiar bodies known as the lobi inferiores (hypoaria) make their appearance at the sides of the infundibulum. They appear to correspond in position with the tuber cinereum of Mammalia[164]. In Birds, Reptiles, and Amphibia the lower part of the embryonic infundibulum becomes atrophied and reduced to a mere finger-like process—the processus infundibuli.
In Mammalia the posterior part of the primitive infundibulum becomes the corpus albicans, which is double in Man and the higher Apes; the ventral part of the posterior wall forms the tuber cinereum. Laterally, at the junction of the optic thalami and infundibulum, there are placed the fibres of the crura cerebri, which are probably derived from the walls of the infundibulum. A special process grows out from the base of the infundibulum, which undergoes peculiar changes, and becomes intimately united with the pituitary body; in which connection it will be more fully described.