THE NERVOUS SYSTEM 24'J 



the median wall of the hemisphere, ending anteriorly in a free 

 branched tip (Fig. 147, ch. PL) 



The principal changes in the telencephalon since the third 

 day comprise: (1) great expansion of the hemispheres and 

 thickening of the ventro-lateral wall to form the corpora striata; 

 (2) origin of the paraphysis which arises as an evagination of the 

 roof just in front of the velum transversum about the middle of 

 the fifth day; (3) formation of the choroid plexus; (4) origin of 

 the anterior commissure within the lamina terminalis; (5) develop- 

 ment of the olfactory region. The general morphology of the 

 adult telencephalon is thus well expressed at this time. 



The Diencephalon has undergone marked changes since the 

 third day. The roof of the parencephalic division has remained 

 very thin, and has expanded into a large irregular sac (Figs. 

 147 and 148), situated between the hinder ends of the hemispheres.. 

 The attachment of the epiphysis has shifted back to the indenta- 

 tion between parencephalic and synencephalic divisions, and the 

 epiphysis itself has grown out into a long, narrow tube, dilated 

 distally, and provided with numerous hollow buds. In the roof 

 of the synencephalic division the posterior commissure has de- 

 veloped (Fig. 147). In the floor the chiasma has become a thick 

 bundle of fibers, and the infundibulum a deep pocket, from the 

 bottom of which a secondary pocket (saccus infundibuli) is grow- 

 ing out in contact with the posterior face of the hypophysis. 

 Following the posterior wall of the infundibulum in its rise, we 

 come to a slight elevation, the rudiment of the mammillary 

 tubercles; just beyond this is a transverse commissure (the in- 

 ferior commissure) ; and the diencephalon ends at the tuberculum 

 posterius. 



The hypophysis has become metamorphosed into a mass of 

 tubules enclosed w^ithin a mesenchymatous sheath; the stalk is 

 continuous with a central tubule representing the original cavity 

 from which the other tubules have branched out (Fig. 148), and 

 it may be followed to the oral epithelium from which the whole 

 structure originally arose. (See note at end of this chapter.) 



The lateral walls of the diencephalon have become immensely 

 thickened, both dorsally and ventrally, and a deep fissure (Fig. 

 147) is found on the inner face at the anterior end, between the 

 dorsal and ventral thickenings. The deepest part of the fissure 

 is a short distance behind the velum transversum; from this a 



