90 THE THIKD DAY. [CHAP. 



to the bulb. On the fourth day the flexure is still greater 

 than on the thii'd, but on the fifth and succeeding days it 

 becomes less obvious owing to the filling up of the parts of the 

 skull. 



Fig. 25. 



Head op a Chick of the third day viewed sideways as a Teansparent 

 Object. (From Huxley.) 



I. a. the vesicle of the cerebral hemisphere. I. 6. the vesicle of the third 

 ventricle (the original fore-brain) ; at its summit is seen the projection of 

 the pineal gland e. 



Below this portion of the brain is seen, in optical section, the optic vesicle a 

 already involuted with its thick inner and thinner outer wall (the letter a is 

 placed on the junction of the two, the primary cavity being almost obliterated). 

 In the centre of the vesicle lies the lens, the shaded portion (represented too 

 large), being the expression of its cavity. Below the lens between the two limba 

 of the horse-shoe is the choroidal fissure. 



II. the mid-brain, now, owing to the cranial flexure, opposite the end of 

 the alimentary canal. III. the hind-brain. V. the rudiments of the fifth 

 cranial nerve, VII. of the seventh. Below tlie seventh nerve is seen the 

 auditory vesicle b. The head having been subjected to pressure, the vesicle 

 appears somewhat distorted as if squeezed out of place. The orifice is not yet 

 quite closed up. 



I. the inferior maxillary process of the first visceral fold. Below, and to 

 the right of this, is seen the first visceral cleft, below that again the second 

 visceral fold (2), and lower down the third (3) and fourth (4) visceral folds. In 

 front of the folds {i.e. to the left) is seen the arterial end of the heart, the 

 aortic arches being buried in their respective visceral folds. 



/. represents at its lowest part the cavity of the alimentary canal as seen 

 through the transparent body of the embryo; at the upper part below the brain 

 it is difficult to distinguish between the transparency due to the presence of the 

 cavity of the alimentary canal, and that caused by the character of the meso- 

 blast at the base of the skull, which, being formed of stellate cells with largel}' 

 developed clear spaces or vacuoles, allows the light to pass readily through it. 

 Near its upper end below the mid-brain is seen a small conical process, the 

 rudiment of the infundibulum. 



9. The two vesicles of the cerebral hemispheres which 



