By the closure of its mouth the pit of involuted
epiblast becomes a completely closed sac with thick
walls and a small central cavity (Fig. 47 B, 1). At
the same time it breaks away from the external epi-


VI.) THE EYE. 135


Fie. 47.
DIAGRAMMATIC SECTIONS ILLUSTRATING THE FORMATION OF
THE Eye. (After Remak.)


In A, the thin superficial epiblast 2 is seen to be thickened at 2,
in front of the optic vesicle, and involuted so as to form
a pit o, the mouth of which has already begun to close in.
Owing to this involution, which forms the rudiment of the
lens, the optic vesicle is doubled in, its front portion 7 being
pushed against the back portion w, and the original cavity
of the vesicle thus reduced in size. The stalk of the vesicle
is shewn as still broad.
In B, the optic vesicle is still further doubled in so as to form a
cup with a posterior wall ~ and an anterior wall 7. In the
hotlow of this cup lies the lens 7, now completely detached
from the superficial epiblast x Its cavity is still shewn.
The cavity of the stalk of the optic vesicle is already much
narrowed.
blast, which forms a continuous layer in front of it,
all traces of the original opening being lost. There is
thus left lying in the cup of the secondary optic vesicle,
an isolated elliptical mass of epiblast. This is the
rudiment of the lens. The small cavity within it
speedily becomes still less by the thickening of the
walls, especially of the hinder one.
