_CH._ Cerebral hemispheres; _F.B._ Vesicle of third ventricle; _M.B._ Mid-brain; _Cb._ Cerebellum; _H.B._ Medulla oblongata; _N._ Nasal pit; _ot._ auditory vesicle in the stage of a pit with the opening not yet closed up; _op._ Optic vesicle, with _l._ lens and _ch.f._ choroidal fissure. The choroidal fissure, though formed entirely underneath the superficial epiblast, is distinctly visible from the outside. _1 F._ The first visceral fold; above it is seen a slight indication of the superior maxillary process. _2_, _3_, _4 F._ Second, third and fourth visceral folds, with the visceral clefts between them.] Owing to the development of the cerebral rudiment the posterior part of the fore-brain no longer occupies the front position (fig. 111, and 112 _FB_), and ceases to be the conspicuous object that it was. Inasmuch as its walls will hereafter be developed into the parts surrounding the so-called third ventricle of the brain, it is known as the vesicle of the third ventricle, or the thalamencephalon. On the summit of the thalamencephalon there may now be seen a small conical projection, the rudiment of the _pineal gland_, while the centre of the floor is produced into a funnel-shaped process, the infundibulum, which, stretching towards the extreme end of the alimentary canal, joins the pituitary body. Beyond an increase in size, which it shares with nearly all parts of the embryo, and the change of position which has already been referred to, the mid-brain undergoes no great alterations during the third day. Its sides will ultimately become developed into the corpora bigemina or optic lobes, its floor will form the crura cerebri, and its cavity will be reduced to the narrow canal known as the iter a tertio ad quartum ventriculum and two diverticula leading from this into the optic lobes. In the hind-brain, or third cerebral vesicle, the roof of the part which lies nearest to the mid-brain, becomes during the third day marked off from the rest by a slight constriction. This distinction, which becomes much more evident later on by a thickening of the walls and roof of the front portion, separates the hind-brain into the cerebellum and the medulla oblongata (fig. 112 _Cb_ and _HB_). While the walls of the cerebellar portion of the hind-brain become very much thickened as well at the roof as at the sides, the roof of the posterior portion or medulla oblongata thins out into a mere membrane, forming a delicate covering to the cavity of the vesicle (fig. 114 _IV_), which here becoming broad and shallow with greatly thickened floor and sides, is known as the fourth ventricle, subsequently overhung by the largely-developed posterior portion of the cerebellum. [FIG. 113. HEAD OF AN EMBRYO CHICK OF THE FOURTH DAY VIEWED AS AN OPAQUE OBJECT: FROM THE FRONT IN A, AND FROM THE SIDE IN B. (Chromic acid preparation.)