9G THE THIRD DAY. [CHAP. 



As the vesicles of the cerebral hemispheres grow out 

 rapidly from the front and under portions of the first cerebral 

 vesicle, they seem to thrust the optic vesicles apart and to 

 the sides. 



Thus these, instead of standing out from the extreme 

 front, come to be placed at the sides of the head, the stalks, 

 which are correspondingly lengthened and narrowed, running 

 obliquely downwards and inwards from the vesicles to open 

 into the cavity of the brain at its base. Their openings are 

 at first placed close to each other at the junction of the 

 cerebral hemispheres with the remnant of the fore-brain (now 

 called the vesicle of the third ventricle), so that the cavities 

 of the two optic vesicles maybe said to communicate directly 

 both with each other and with the cavities of the cerebral 

 hemispheres. The later connection is however soon lost, and 

 the stalks of the optic vesicles then open exclusively into the 

 third ventricle. At the same time the floor of the third ven- 

 tricle, during the occurrence of the cranial flexure, grows 

 down and thrusts apart the openings of the two optic stalks. 

 At a later date the stalks shift their position backwards, and 

 thus become connected chiefly with the base of the mid- 

 brain. 



While these changes have been going on in the optic 

 stalks, development has also proceeded in the region of 

 the vesicles themselves, and given rise to the rudiments of 

 the retina, lens, vitreous humour, and other parts of the eye. 



The changes through which these are formed are of a 

 somewhat complicated character, and not a few points in 

 reference to them are still involved in some doubt. 



Towards the end of the second day, the external or super- 

 ficial epiblast which covers, and is in all but immediate 

 contact with the most projecting portion of the optic vesicle, 

 becomes thickened. This thickened portion is then driven 

 inwards in the form of a shallow open pit with thick walls 

 (Fig. 27 A. x), carrying before it the front wall (?•) of the optic 

 vesicle. To such an extent does this involution of the super- 

 ficial epiblast take place, that the front wall of the optic 

 vesicle is pushed close up to the hind wall, and the cavity of 

 the vesicle becomes almost obliterated (Fig. 27, B). 



The bulb of the optic vesicle is thus converted into a cup 

 with double walls, containing in its cavity the portion of 



