198 MR. H. GRAY ON THE DEVELOPMENT OF THE RETINA AND OPTIC NERVE, 
existing and separating it from the vestibule itself has increased very considerably, 
so that now a well-marked separation is seen to exist, although both cavities com- 
municate with each other. A slight depression or shallow pit is now observed to 
exist at the end of the terminal vesicle, which soon becomes very distinct. This I 
believe indicates the first rudiment of one of the semicircular canals ; no trace of the 
folding-in or depression of the integument in this situation could be observed. 
About the sixth day this depression is more marked, and it is apparently from the 
amalgamation of this reflected portion and the inner wall of the vesicle with each 
other, that the membranous semicircular canals are formed. At first they appear to 
retain the same diameter throughout, but between the twelfth and thirteenth days 
they become somewhat contracted in parts, leaving some portions of their original 
diameter : it is these that apparently ultimately form the ampullae. At about the 
same period a cartilaginous nidus is deposited on the outer side of the membranous 
labyrinth, which is soon developed into the various parts of the osseous labyrinth ; 
at the same time also a small quantity of calcareous matter is deposited in the vesti- 
bular sac. It is interesting to observe that the membranous labyrinth between the 
eighth and thirteenth days has a structure almost precisely similar to that of the 
retinal expansion of the same period, consisting, like it, of a distinct but very delicate 
fibrous mesh, in the spaces between which are deposited a quantity of granular matter 
and numerous nucleated cells, its exterior surface being composed of a dense mass of 
nuclei, almost precisely analogous to the '‘agglomerated granules” which form so 
large a portion of the entire substance of the retina. From the preceding observa- 
tions the following conclusions may be drawn. 
That the membranous labyrinth, like the retina, is a protruded portion of the brain, 
being hollow and communicating with the ventricular cavity from which it arises. 
As the progressive development of the brain proceeds, the auditory sac becomes 
more elongated and of a pyriform shape, the dilated portion being analogous to the 
vestibule, the contracted tubular portion to the auditory nerve ; this subsequently 
becomes solid, and the cavity in the vestibule does not then communicate with the 
ventricle, from the wall of which it is an offset. This representative of the normal 
condition of the organ in some of the lower animals now takes on a higher form of 
development, for the membranous semicircular canals are now added to the vestibule, 
being formed by a contraction, and subsequently, a folding inwards and union of a 
portion of the walls of the vesicle itself ; lastly, the walls of these canals becoming in 
parts contracted, the dilated ampullee are formed. 
