AND OF THE MEMBRANOUS LABYRINTH AND AUDITORY NERVE. 
193 
what was seen on the third day ; it had become more spherical in shape and increased 
in size ; it now appeared to commence at the margin of the lens, which was situated 
at the more anterior part of the eye ; it could not be traced around it, as in some of 
the preceding observations, but only overlapped it slightly by a thin beveled border, 
so that it appeared that the most external part of the original protruded vesicle had 
become absorbed to complete the formation of this rapidly developing membrane. 
The tubular portion of the original protrusion (the optic nerve), from the great deve- 
lopment of the cerebral mass, was hidden from view. 
On the fourth day (fig. 10) the eye had become more spherical in form, the outer 
envelope was distinct and darkly granular ; it appeared now to terminate at the edge 
of the lens, whilst a paler line could be traced over this body, an evident indication 
of the formation of the sclerotic and cornea. The retina was now less distinctly 
marked on account of the dark granular tinge beneath the outer envelope ; from the 
formation of the pigment in the epithelial cells of the choroid, it could now only be 
traced to the edge of the lens, not overlapping it as in the last examination: a distinct 
fissure was still perceptible on the under surface. 
On the fifth day (fig. 11) the eye had greatly increased in size ; and along its under 
surface the fissure was seen running, from its anterior part at the margin of the lens, 
to a line which bordered the back part of the sclerotic ; the capsule of the lens was 
distinctly formed and separate from the lens itself. Both optic nerves were seen; they 
were tubular in form and presented a pale granular appearance ; they passed inwards 
in the direction of the under surface of the corpora quadrigemina, but they were not 
united together in the chiasma until the seventh day. At this period, on making a 
vertical section of the organ, the retina was to the naked eye distinctly observed to 
arise from the margin of the lens, and could be detached from the other membranes 
as a perfectly distinct and separate layer. 
From these observations it is seen that the retina is originally a protrusion from 
the anterior cerebral cell, being hollow and communicating with its cavity ; that as 
the progressive development of the brain takes place, the optic vesicle becomes more 
separated from its parent cell and assumes a pyriform shape, presenting a dilated ex- 
tremity, the future retina, and a tubular portion (the optic nerve). In progress of de- 
velopment a fissure is observed on its under surface, which is evidently connected 
with the separation which is effected between the ocular vesicle and the tubular 
pedicle (the optic nerve) which connects it with the cerebral cell. As this tubular 
prolongation becomes solidified so as to form the optic nerve, no communication can 
be traced between the optic vesicle and the cavity, from which it is an offset. By de- 
grees the spherical end of the protrusion is absorbed, and the retina, now fully 
formed, becomes attached to the margin of the lens, having previously completely 
surrounded that body. The optic nerve can then be seen not only to be connected 
to the anterior cerebral cell, but uniting with its fellow at the under surface of the 
optic lobes, is seen partly to terminate in those bodies, 
2 c 
MDCCCL. 
