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. 



kth. 



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 20 s 

 stage (Fig. 85), and grows rapidly inward in contact with the 

 floor of the diencephalon. At about the 30 s 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. 

 88). At about the 36 s stage its distal portion flattens laterally 



