122 DEVELOPMENT OF TI1E FROG 



larger portion, the fore-brain, BF, which is produced laterally 

 into a pair of hollow outgrowths, the optic vesicles. 



The further development of the brain is illustrated by Figs. 

 28 and 29. It will be seen that the rectangular bending of the 

 brain, which is known as the cranial flexure, and which was so 

 prominent a feature in the earlier stage, is no longer obvious ; 

 a closer comparison of the figures will show, however, that this 

 straightening of the brain, or rectification of the cranial flexure, 

 is apparent rather than real, and is brought about partly by the 

 development of the cerebral hemispheres, which grow upwards 

 and forwards from the fore-brain, and still more largely by the 

 formation of the mouth and the growth forwards of the face 

 and lips, which cause the brain to take a much less prominent 

 share in determining the shape of the head. 



The hind-brain, BH, has undergone but little change in 

 Fig. 28, except an increase in thickness of its floor and sides. 

 At the stage represented in Fig. 29 it is separated from the 

 mid-brain on the dorsal surface by a well-marked groove, im- 

 mediately behind which the roof of the hind-brain is thickened 

 transversely to form the cerebellum, CB. The cavity of the 

 hind-brain remains as the fourth ventricle, the roof of which 

 is very thin and thrown into numerous transverse folds, C P, 

 which hang down into the ventricle, and between the layers of 

 which lie the bloodvessels of the choroid plexus of the ventricle. 



The mid-brain, BM, thickens on its floor to form the crura 

 cerebri. Its roof grows out laterally into a pair of hollow ovoid 

 processes, the optic lobes ; and its cavity persists as the 

 aqueductus Sylvii, or iter a tertio ad quartum ventriculum. 



The fore -brain, BF, becomes the tnalamencephalon of the 

 adult ; its cavity becomes the third ventricle, which by thick- 

 ening of its walls to form the optic tnalami is reduced to a 

 vertical cleft, very narrow from side to side. Its floor is pro- 

 duced downwards and backwards into a hollow sac-like diverti- 

 culum, the infundibulum, I, in connection with which is the 

 pituitary body. In front of the infundibulum is a transverse 

 ridge projecting into the ventricle, and formed by the roots of 

 the optic nerves. 



The roof of the fore-brain remains thin ; a little behind the 

 middle of its length the pineal body, PN, arises as a median 

 hollow diverticulum, Figs. 28 and 29 ; this is formed at the spot 

 where the final closure of the neural tube took place, and is at 



