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. 
(b) 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 145 stage, 
and it is probable that it involves only the dorsal zones. It is, 
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Fia. 84. — Transverse section through the fore-brain and optic vesicles of a 
16-s embryo. 
Am. F., Amniotie 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 20s stage, when a slight transverse fold or indentation 
in the roof (velum transversum) 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 indefiniteness 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 
