STRUCTURE OF FOUR-DAY CHICKS II7 



come appreciably deepened and lies in close proximity to 

 Rathke's pocket with which it is destined to fuse in the forma- 

 tion of the hypophysis (Fig. 43). Later in development the 

 lateral walls of the diencephalon become greatly thickened to 

 form the thalami, thus reducing the size and changing the 

 shape of the diocoele, which is known in adult anatomy as the 

 third brain ventricle. The anterior part of the roof of the 

 diencephalon remains thin and by the ingrowth of blood vessels 

 from above is pushed into the third ventricle to form the an- 

 terior choroid plexus. 



The boundary between the diencephalon and the mesen- 

 cephalon is an imaginary line drawn from the internal ridge 

 formed by the original dorsal constriction between the primary 

 fore-brain and mid-brain, to the tuberculum posterius (Fig. 

 42, A). The tuberculum posterius is a rounded elevation in 

 the floor of the brain of importance chiefly because it is regarded 

 as marking the boundary between diencephalon and mesen- 

 cephalon. 



The Mesencephalon. — The mesencephalon as yet shows no 

 specializations, beyond a thickening of its walls. The dorsal 

 and lateral walls of the mesencephalon later increase rapidly 

 in thickness and become the optic lobes (copora quadrigemina) 

 of the adult brain. The optic lobes should not be confused with 

 the optic vesicles arising from the diencephalon of the embryo. 

 They are entirely different structures. The floor of the mesen- 

 cephalon also becomes greatly thickened and is known in the 

 adult as the crura cerebri. It serves as the main pathway of the 

 fiber tracts which connect the cerebral hemispheres with the 

 posterior part of the brain and the spinal cord. The originally 

 capacious mesocoele is thus reduced by the thickenmg of the 

 walls about it to a narrow canal (Aqueduct of Sylvius). 



The Metencephalon. — The boundary between the mesen- 

 cephalon and metencephalon is indicated by the original inter- 

 neuromeric constriction which separated them at the time of 

 their establishment (Cf. Figs. 20 and 42). The caudal boun- 

 dary of the metencephalon is not definitely defined. It is 

 regarded as being located approximately at the point where 

 the brain roof changes from the thickened condition character- 

 istic of the metencephalon to the thin condition characteristic 

 of the myelencephalon. The metencephalon shows practically 



