FROM TWELVE TO THIRTY-SIX SOMITES 167 
forms an epithelial sac, which at first practically fills the cavity 
of the optic cup. However, it very soon becomes detached from 
the posterior wall of the optic cup, which expands with great 
rapidity, and the lens is left at the mouth of the cup. The walls 
yer of optic 
mber of the 
alls of the optic 
y; the lens-sac is 
The w 
op. c. i., Inner la 
93). 
ao C 
gf. 
ral wall of diencephalon. 
p. Ch., Posterior cha 
‘i 
DMs Wate 
op. St., Optic stalk. 
w of the head (compare I 
; but the lens is represented as a solid bod 
ctoderm of the surface of the head. 
ch. Fis., Choroid fissure. 
Fic. 96. — Semi-diagrammatic representation of the eye of an embryo of about 27-30 s. 
ot 
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meee Lael 
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[eb) =< ©) 
o = 2) 
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ao IS) a er 
Lan! orm omy — 
Sept a 5 ~ 
nm Cd . 
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nao Qo ne 4 
@ + SOoOHRR., 
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os FS emt 
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— 4 & OgomM ~ 6 
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do mt 
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TM 
Se 
of the lens sac are at first of practically even thickness (2 
but by the 35s stage a great difference has arisen by the elonga- 
tion of the cells of the inner wall, which are destined to form 
lens fibers: the cells of the anterior (outer) wall elongate much 
