9H 



numerous faint longitudinal strite. On the eighteenth day this 

 membrane when separated from the other layers is seen composed 

 of numerous fibri Hated meshes, in which are deposited the nucleated 

 vesicles which are formed as early as the eighth day. From these 

 observations it is seen that the retina is formed as a protrusion from 

 the most anterior cerebral cell, being hollow and communicating with 

 its cavity ; that it subsequently assumes a pyriform shape, presenting 

 a dilated end, the future retina, and a tubular portion, the optic 

 nerve. As the tubular portion becomes solidified so as to form the 

 optic nerve, then no communication can be traced between the op- 

 tic vesicle and the cavity from v/hich it is an offset. By degrees 

 the spherical end of the protrusion is absorbed, and the retina, being 

 now fully formed, becomes attached to the margin of the lens. The 

 optic nerve is then traced to be connected not only with 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. The deduc- 

 tions from these observations may be thus briefly stated : — 



1st. They confirm the observations on the structure of the retina 

 made by Bowman, who has shown that the essential part of this 

 membrane is analogous to the cineritious matter of the brain, and is 

 composed like it of a fibrous mesh intermingled with vesicles of 

 grey matter, being, in fact, a portion of the cerebrum pushed out- 

 wards and connected with the brain by its appropriate commissure, 

 the optic nerve. The mode of development of this membrane 

 •would show this to be the correct view of the structure of this 

 essential part of the retinal expansion, and at the same time disprove 

 the statements of Henle, who believed it to be more analogous to 

 epithelium, 



2nd. The origin of the optic vesicle from the anterior cerebral 

 cell, would show the incorrectness of the opinion of those anato- 

 mists who have stated that none of the fibres of the optic nerve 

 could be traced to the optic thalanii. The thalami being developed 

 from the same centre from whence these vesicles arise, would render 

 it exceedingly probable that the optic nerves had some connection 

 with those bodies. 



The second part of the paper describes the development of the 

 membranous labyrinth and auditory nerve. 



The essential part of the auditory apparatus, viz. the membranous 

 labyrinth, consists, like the retina, of a membranous lamina formed 

 of the terminal axes cylinders of the nerve tubules in intimate con- 

 nection with a layer of closely-set nucleated cells ; like it also, it 

 may be regarded as a portion of the brain protruded outwards, and 

 connected with an appropriate apparatus which receives and trans- 

 mits its peculiar impressions ; its mode of development also shows a 

 striking analogy between it and the retinal expansion. 



At the fiftieth hour of incubation, there is seen on either side of 

 the medulla oblongata, (which is not closed in above and presents 

 an open shallow cavit}^,) the first rudiment of the auditory sac, in 

 the form of a small circular-shaped protruded vesicle, communi- 

 cating with the ventricular cavity from the lateral wall of which it 



