AVES. 



175 



dilated, and may be distinguished from the posterior part as the 

 unpaired rudiment of the cerebral hemispheres. It soon bulges 

 out laterally into two lobes, which do not however become 

 separated by a median partition till a much later period. 



Owing to the development of the 

 cerebral rudiment the posterior part 

 of the fore-brain no longer occupies 

 the front position (fig. in, and 112 

 FB\ and ceases to be the conspicu- 

 ous 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 thalamen- 

 cephalon there may now be seen a 

 small conical projection, the rudi- 

 ment of the pineal gland, while the 

 centre of the floor is produced into a 

 funnel-shaped process, the infundi- 

 bulum, 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 



FIG. 112. SIDE VIEW OF THE 

 HEAD OF AN EMBRYO CHICK OF 

 THE THIRD DAY AS AN OPAQUE 

 OBJECT. (Chromic acid prepa- 

 ration.) 



CH. Cerebral hemispheres ; 

 F.B. Vesicle of third ventricle ; 

 M.B. Mid-brain; Cb. Cerebel- 

 lum; 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 /. lens and 

 ch.f. choroidal fissure. The cho- 

 roidal fissure, though formed en- 

 tirely underneath the superficial 

 epiblast, is distinctly visible from 

 the outside. 



i F. The first visceral fold ; 

 above it is seen a slight indica- 

 tion of the superior maxillary 

 process. 



2, 3, 4 F. Second, third and 

 fourth visceral folds, with the 

 visceral clefts between them. 



