n6 



EARLY EMBRYOLOGY OF THE CHICK 



(Fig. 42, A). Velum transversum is the name given to the 

 internal ridge formed by the deepening of the dorsal constriction 

 which was first noted in chicks of 55 hours as indicating the 

 impending division of the primary fore-brain (Fig. 35). The 

 recessus opticus is a transverse furrow in the floor of the brain 

 which in the embryo leads on either side into the lumina of the 

 optic stalks. 



The Diencephalon. The lateral walls of the diencephalon at 

 this stage show little differentiation except ventrally where the 



mandibular arch 



Seessell's pocket 



Rathke's pocket 



tnesonephros 

 somite 

 coelom 



dorsal aorta 



notochord 



neural tu 



somite 

 allantoic stalk 



proctodaeum 

 post- anal gut 

 cloaca 



' splanchnopleure 

 of yolk sac 



FIG. 43 . Diagram of median longitudinal section of four-day chick. Due 

 to a slight bend in the embryo the section is para-sagittal in the mid-dorsal 

 region but for the most part it passes through the embryo in the sagittal plane. 



optic stalks merge into the walls of the brain. The develop- 

 ment of the epiphysis as a median evagination in the roof of the 

 diencephalon has already been mentioned (Chap. XII). Ex- 

 cept for some elongation it does not differ from its condition 

 when first formed in embryos of about 55 hours. The in- 

 fundibular depression in the floor of the diencephalon has be- 



