A VES. 



145 



The cranial fiexure progresses rapidly, the front-brain being more 

 and more folded down till, at the end of the third day, it is no longer 

 the first vesicle or fore-brain, but the second cerebral vesicle or mid- 

 brain, which occupies the extreme front of the long axis of the embryo. 

 In fact a straight line through the long axis of the embryo would 

 now pass through the mid-brain instead of, as at the beginning of 

 the second day, through the fore-brain, so completely has the front 

 end of the neural canal been folded over the end of the notochord. 

 The commencement of this cranial flexure gives the body of an embryo 

 of the third day somewhat the appearance of a chemist's retort, the 

 head of the embryo corresponding to the bulb. On the fourth day 

 the flexure is still greater than on the third, but on the fifth and 

 succeeding days it becomes less obvious. 



The anterior part of the fore-brain 

 has now become greatly dilated, and may 

 be distinguished from the posterior part 

 as the unpaired rudiment of the cere- 

 bral 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. ] 11, 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 ven- 

 tricle, or the thalamencephalon. 



On the summit of the thalamen- 

 cephalon 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 to- 

 wards 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 em- 

 bryo, 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. 



Fig. 112. Side view of the 



HEAD OF AN EmBRTO ChICK OF THE 

 THIBD DAY AS AN OPAQUE OBJECT. 



(Chromic acid preparation.) 



CH. Cerebral hemispheres ; 

 F.B. Vesicle of third ventricle; 

 M. 5. Mid-brain ; CZ>. Cerebellum; 

 H.B. Medulla oblongata ; N. Na- 

 sal pit : ot. auditory vesicle in the 

 stage of a pit ■with the opening 

 not yet closed up ; op. Optic ves- 

 icle, with I. lens and ch.f. cho- 

 roidal fissure. The choroidal fis- 

 sure, though formed entirely un- 

 derneath the superficial epiblast, 

 is distinctly visible from the out- 

 side. 



1 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. 



B. E. II. 



10 



