154 THE DEVELOPMENT OF THE. CHICK 
conspicuous thickening, the optic chiasma, which is continued 
as a ridge in the lateral ventral zones on each side (Fig. 86). 
The infundibulum follows just behind this, and constitutes a 
considerable pouch-shaped depression from which the saccus 
infundibuli grows out later. The posterior wall of this depression 
rises sharply and joins the thickened tuberculum posterius which 
is the end of the floor of the diencephalon. The diencephalon is 
compressed laterally (Fig. 97); the dorsal zones are slightly 
thickened, indicating the future thalami optici. 
spoFenc- 
Me Lé 
es 
pent SD posse 
3 Mesenc 
Synenc 
PATONC. 
LD. 
Fig. 88. — Optical longitudinal section of the head of an embryo of 39 s. 
Abbreviations as before. 
The hypophysis should be mentioned here, although it is not 
embryologically a part of the brain. It arises as a median tubu- 
lar invagination of the ectoderm of the ventral surface of the 
head immediately in front of the oral plate at about the 20s 
stage (Fig. 85), and grows rapidly inward in contact with the 
floor of the diencephalon. At about the 30s stage its end 
reaches nearly to the infundibulum (Fig. 87). At first part of 
its wall is formed by the oral plate, and when this ruptures the 
effect is to shorten the apparent length of the hypophysis (Fig. 
SS). At about the 36s stage its distal portion flattens laterally 
