AND OF THE MEMBRANOUS LABYRINTH AND AUDITORY NERVE. 
195 
posed of a very fine pale granular lamina, marked by numerous faint longitudinal 
striae : on the eighteenth, this lamina, when separated from the rest of the membrane, 
is composed of numerous fibrillated bundles, which interlacing with each other form 
numerous meshes, in which are deposited the gray nucleated vesicles, which are seen 
to be formed as early as the eighth day. On the twenty-first day this lamina presents 
the same structure as is found in the full-grown bird. The preceding observations can, 
I think, be applied in explanation of some lately disputed points in connection with 
the anatomy and physiology of the retina and optic nerve. And first of the retina. 
This membrane has been seen to arise as a protrusion from the cells forming the 
future brain. Now in the microscopic anatomy of this membrane, as described by 
Bowman, the vesicular layer of the retina is stated to be analogous to the vesicular 
layer of the hemisphere of the brain, whilst Henle, and other anatomists of great 
note, consider the elements of this layer to be more analogous to epithelium. From 
the circumstance of the retina arising, as it undoubtedly does, from the cerebral cells, 
being in fact part of them and performing a similar function, we have, I think, a great 
proof of the similarity in the structure of these two parts. There is another point 
also, I think, of some importance in proving, as it does, that the opinion of some 
modern anatomists is incorrect in stating that none of the fibres of the optic nerve 
can be traced to the optic thalamus. I have said that the origin of the retina is from 
the anterior cerebral cell, and that at a future period the optic nerve could be traced 
uniting with its fellow of the opposite side beneath the optic lobes. It is in the 
anterior cerebral cell that the thalami are subsequently formed, which makes it ex- 
ceedingly probable that some of its fibres are connected with it, although the greater 
majority may be traced to the optic lobes. 
These facts I think are of some importance, and prove how deductions formed 
from microscopic embryology may be applied to confirm dissections or microscopic 
investigations made on the same parts in the mature animal. 
On the Development of the Membranous Labyrinth and Auditory Nerve. 
The observations I made prior to my description of the mode of evolution of the 
retina will almost apply here as a preface to my observations on the development of 
the membranous labyrinth and auditory nerve, for the essential part of this mem- 
brane consists, like the retina, of a fibrous lamina formed of the terminal axes, 
cylinders of the nerve tubules or of terminal loops of nerves, which are in intimate 
relation with a layer of dark and closely-set nucleated cells, not unlike those found 
lying between the meshes of the fibrous lamina of the retina; like it also, it may be 
regarded (as the following observations will show) as a protruded portion of the 
brain, modified somewhat in its texture and connected with an appropriate apparatus, 
which receives and transmits its peculiar impressions. 
The following observations are intended to demonstrate the mode of development 
of this membrane, and they will, I think, confirm, not only the description given of 
2 c 2 
