118 THE THIRD DAY. [CHAP. 



FIG. 38. 



HEAD OF A CHICK OF THE THIRD DAY VIEWED SIDEWAYS AS A 

 TRANSPARENT OBJECT. (From Huxley.) 



I a. the vesicle of the cerebral hemisphere. 1 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 being the expression of 

 its cavity. Below the lens between the two limbs of the horse- 

 shoe is the choroidal fissure. 



II. the mid-brain. III. the hind-brain. V. the rudiments of 

 the fifth cranial nerve, VII. of the seventh. Below the 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 or man- 

 dibular 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 r 

 the aortic arches being buried in their respective visceral folds. 



f. represents the mesoblast of the base of the brain and spinal 

 cord. 



