STRUCTURE OF FIFTY-HOUR CHICKS 95 



II. THE NERVOUS SYSTEM 



Growth of the Telencephalic Region. The completion of 

 torsion in the head region causes rapid changes in the configura- 

 tion of the brain as seen in entire embryos from 40 to 50 hours 

 of incubation. The same fundamental regions can, however, 

 be identified throughout this range of development. The an- 

 terior part of the brain has undergone rapid enlargement. A 

 slight constriction in the dorsal wall (Fig. 35) indicates the 

 impending division of the prosencephalon into telencephalon 

 and diencephalon. Except for its considerable increase in size 

 no important changes have taken place in the telencephalic 

 region. 



The Epiphysis. In the mid-dorsal wall of the diencephalic 

 region a small evagination has appeared. This evagination is 

 the epiphysis (Fig. 34 and 35). It is destined to become dif- 

 ferentiated into the pineal gland of the adult. 



The Infundibulum and Rathke's Pocket. In the floor of the 

 diencephalon the infundibular depression has become deepened 

 and lies close to a newly formed ectodermal invagination known 

 as Rathke's pocket (Fig. 35). The epithelium of Rathke's 

 pocket is destined to be separated from the superficial ectoderm 

 and to become permanently associated with the infundibular 

 portion of the diencephalon to form the hypophysis or pituitary 

 body. 



The Optic Vesicles. The optic vesicles have undergone 

 changes which completely alter their appearance. In 33-hour 

 chicks they are spheroidal vesicles connected by broad stalks 

 with the lateral walls of the diencephalon (Fig. 21). At this 

 stage the lumen of each optic vesicle (opticcele) is widely con- 

 tinuous with the lumen of the prosencephalon (prosoccele) 

 (Fig. 28, A). The constriction of the optic stalk which begins 

 to be apparent in 38-hour embryos (Fig. 22) is much more 

 marked in 55-hour chicks. 



The most striking and important advance in their develop- 

 ment is the invagination of the distal ends of the single-walled 

 optic vesicles to form double walled optic cups (Fig. 36, B). 

 The concavities of the cups are directed laterally. Mesially 

 the cups are continuous with the ventro-lateral walls of the 

 diencephalic region of the original prosencephalon over the 



