THE NERVOUS SYSTEM 249 
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; (8) 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 within 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. 
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 
