376 THE BABBIT. 



blast outside the brain, and give rise to the choroid plexus of 

 the third ventricle. 



The floor of the thalainencephalon also remains thin, though 

 not so thin as the roof. The anterior part of the floor is crossed 

 by a shallow transverse groove, which is prolonged outwards 

 into the optic stalks (Figs. 150, 151, and 155). The posterior 

 part of the floor gives rise to the infundibuhim. This is a 

 median, thin- walled depression, from the hinder end of which a 

 hollow finger-like diverticulum arises on the tenth day (cf. Fig. 

 150, in); this diverticulum lies, from the first, in very close 

 relation with the anterior end of the notochord, and with the 

 pocket-like outgrowth from the stomatodamm (Fig. 150, it), 

 which gives rise to the pituitary body. This anterior end of the 

 notochord is ultimately absorbed and obliterated ; but the infun- 

 dibular diverticulum and the pituitary body remain in intimate 

 relation throughout life ; the diverticulum forming what is spoken 

 of, in the adult rabbit, as the posterior lobe of the pituitary body. 



The further development of the pituitary body itself, which may 

 conveniently be dealt with here, is as follows. The stomatodreal 

 diverticulum (Fig. 150, pt) dilates at its blind end, and gives 

 off from this terminal dilatation hollow outgrowths ; these branch 

 freely (Fig. 151, pt), but in the later stages of development 

 become solid. The central dilated cavity of the pituitary body 

 persists; it retains its communication with the buccal cavity, 

 through the tubular stalk, for some time. The formation of the 

 palate (Fig. 151, pl) leaves the pituitary stalk in communica- 

 tion with the narial passage, but cuts it off from direct com- 

 munication with the buccal cavity. In the later stages, the 

 pituitary stalk loses its connection with the narial passage, and 

 becomes obliterated. 



The cerebral hemispheres arise, as in the chick, in the first in- 

 stance as a median anterior prolongation of the thalamencephalon, 

 which may be termed the vesicle of the hemispheres. This soon 

 becomes divided by an inwardly projecting fold of its anterior 

 wall into right and left lobes, which by further growth become 

 the cerebral hemispheres ; the median anterior wall of the vesicle, 

 between the bases of the hemispheres, persisting as the lamina 

 terminalis. 



From their mode of formation the hemispheres are necessarily 



