512 EMBRYOLOGY OF THE CHICK 



All discussion of the central nervous system in our future study of 

 comparative anatomy will depend upon the student's thorough under- 

 standing- of the development of the brain regions and vesicles as here 

 discussed. Consequently, the various arbitrary lines used as demarca- 

 tions must be carefully studied. 



The division between telencephalon and diencephalon is the imag- 

 inary line drawn from the velum transversum to the recessus opticus. 

 The velum is that slight extension marking the point where the primary 

 fore-brain is to divide, while the recessus is that transverse furrow in 

 the floor of the brain which leads directly into the lumina of the optic- 

 stalks. 



The Diencephalon: There is little change in this on the fourth day, 

 except that the infundibular depression in the diencephalon has deep- 

 ened, and lies close to Rathke's pocket (Fig. 301, I), with which it later 

 fuses to form the hypophysis. Later the lateral walls of the diencepha- 

 lon are to become thickened to form the thalami. As these thalami 

 grow inward toward each other, they will cause the diocoele or third 

 ventricle to become quite small. The anterior part of the roof of the 

 diencephalon remains thin, and the blood-vessels grow downward into 

 the diocoele as the choroid plexus. 



The division between diencephalon and mesencephalon is an imag- 

 inary line drawn between the tuberculum posterius (a rounded elevation 

 in the floor of the brain, of importance only as a landmark of this kind), 

 and the internal ridge formed by the original dorsal constriction between 

 the primary fore-brain and mid-brain. 



The Mesencephalon: There is little change in this portion of the 

 brain, though a little later, dorsal and lateral walls become thickened 

 to form either the optic lobes or the corpora quadrigemina. Optic lobes 

 and optic vesicles must not be confused, as these are two separate and 

 distinct structures. 



The floor of the mesencephalon thickens to form the cerebral pedun- 

 cles of the adult, which serve as the main pathway for the fiber tracts 

 which connect the cerebral hemispheres with the posterior part of the 

 brain and spinal cord. The mesocoele becomes quite small by these 

 various thickenings and is now called the aqueduct of Sylvius. 



The Metencephalon: The metencephalon is separated from the 

 mesencephalon by the original inter-neuromeric constrictions which 

 arose early and marked off this portion of the brain. The caudal boun- 

 dary is not well defined, though it is supposed to merge in the myelen- 

 cephalon where the roof changes from its thickened state to the thinner 

 condition observed more posteriorly. There is little change on the 

 fourth day in this region, though later an extensive ingrowth of fiber 

 tracts develops both on the ventral and lateral walls. These fiber tracts 

 form the pons and the cerebellar peduncles, while the roof of the meten- 

 cephalon enlarges and becomes the cerebellum. 



