94 THE THIRD DAY. [CHAP, 



known as the fourth ventricle, subsequently overhung by the 

 largely developed posterior portion of the cerebellum. 



The third day, therefore, marks the distinct differentiation 

 of the brain into its fundamental parts: the cerebral hemi- 

 spheres, the central masses round the third ventricle, the 

 corpora bigemina, the cerebellum and the medulla oblongata j 

 the original cavity of the neural canal at the same time, 

 passing from its temporary division of three single cavities 

 into the permanent arrangement of a series of connected 

 ventricles, viz. the lateral ventricles, the third ventricle, the 

 iter (with a prolongation into the optic lobe on each side), 

 and the fourth ventricle. 



10. At the same time that the outward external shape 

 of the brain is thus being moulded, internal changes are 

 taking place in the whole neural canal. These are best seen 

 in sections. 



At its first formation, the section of the cavity of the 

 neural canal is round or nearly so. 



About this time, however, the lining of involuted epiblast 

 along the length of the whole spinal cord becomes very much 

 thickened at either side, while increasing but little at the 

 mid-points above and below. The result of this is that the 

 cavity as seen in section (Fig. 44), instead of being circular, 

 has become a narrow vertical slit, almost completely filled in 

 on either side. 



In the region of the brain the thickening of the lining 

 epiblast follows a somewhat different course. While almost 

 everywhere the sides and floor of the canal are greatly thick- 

 ened, the roof in the region of the various ventricles, not of 

 the fourth only, but of the others as well, becomes excessively 

 thin, so as to form a membrane reduced to almost a single 

 layer of cells. (Fig. 20. iv.) 



11. In the preceding chapter we saw how the first cere- 

 bral vesicle, by means of lateral outgrowths followed by 

 constrictions, gave rise to the optic vesicles. These and the 

 parts surrounding them undergo on the thir4 day changes 

 which result in the formation of the eyeball. 



At their first appearance the optic vesicles stand out at 

 nearly right angles to the long axis of the embryo (Fig. 15), 

 and the stalks which connect them with the fore-brain are' 

 short and wide. We have already said (p. 77) that the con- 



