150 THE DEVELOPMENT OF THE CHICK 



diameter of the fore-brain itself, though this is relatively slight 

 up to twenty somites, but it enhances the general effect of the 

 change in position of the optic stalk. The subsequent history 

 of the optic vesicles is given beyond. 



(6) The delimitation of the tel- and diencephalon is initiated 

 by a forward expansion of the anterior end of the primary fore- 

 brain, which becomes the telencephalon or secondary fore-brain, 

 the remainder being then known as the diencephalon or 'tween 

 brain. The expansion proceeds very rapidly from the 14 s stage, 

 and it is probable that it involves only the dorsal zones. It is, 



Ectam . 



E.E.B.C. 



so'pf. 



-"? 



* 



opl-sf. 



FIG. 84. Transverse section through the fore-brain and optic vesicles of a 

 16-s embryo. ^ 



Am. F., Amniotic fold. Ectam., Ectamnion. L., Left side, op.st., 

 Optic stalk. R., Right side. Other abbreviations as before. 



however, difficult to establish an exact line of demarcation be- 

 tween the two subdivisions of the primary fore-brain, until about 

 the 18 to 20 s stage, when a slight transverse fold or indentation 

 in the roof (velum trans versum) gives a dorsal landmark (Figs. 

 73, 85); the recessus opticus forms the ventral boundary between 

 the two. The velum transversum lies a considerable distance 

 above the dorsal end of the lamina terminalis, but it is difficult 

 to say just how far, owing to the indefmiteness of this point for 

 some time after the disappearance of the neuropore. A line 

 drawn between the velum transversum and the recessus opticus 

 may be taken as the boundary between the two divisions of the 



