AND OF THE MEMBRANOUS LABYRINTH AND AUDITORY NERVE. 
197 
aperture of communication is however much smaller, having become more contracted, 
and this contraction apparently increases as the separation between the auditory 
vesicle and its parent cell takes place. At the sixty-fifth hour (see fig. 15) the ear- 
bulb has increased considerably in size, and a more marked separation is now seen 
to exist between the auditory nerve and the expanded vestibular sac ; the latter has 
assumed an oval shape and is now directed slightly backwards, the cavity in its in- 
terior still existing ; but the auditory nerve has become now quite solidified, so that 
no communication exists at this period between it and the ventricular cavity. From 
the description which I have already given, a marked similarity may be observed be- 
tween the origin of this membrane and that of the retina and optic nerve ; these parts 
arise in both cases as a protruded portion of the cerebral mass, being hollow and 
communicating with the cavity of the parent cell ; in process of time a gradual sepa- 
ration takes place between them and the parts from which they arise; they then assume 
a pyriform shape, but still communicate with the cerebral cavity ; as however the 
nerve becomes solidified and more fully formed, and the separation between them is 
more fully effected, then no communication can be traced between the two cavities. 
It is in this stage of the development of the auditory apparatus in the Bird, that a 
remarkable similarity is to be observed between it and the normal condition of the 
same part in some of the lower animals. There are in fact now formed the two 
elementary portions of this apparatus, the auditory nerve and its vesicular bulb (the 
analogue of the vestibular sac). Such is the simple condition of this organ in the 
Crustacea and in the cephalopod mollusks. 
I shall, in the next place, proceed to describe the observations I have made on the 
development of the semicircular canals, or rather of those portions of the mem- 
branous labyrinth which line those cavities, and which are found superadded in most 
Birds, Fishes, and Mammalia. 
At the seventy-second hour (fig. 16) the vestibular sac has become more distinctly 
separated from the cavity from which it originated, but is connected with it by the 
auditory nerve, which is fully formed and of large size ; the vesicle is still quite hollow, 
and contains a thin limpid fluid ; but its oval form is lost from a distinct contraction 
of its (apparent) entire circumference, which is observed about its centre ; this con- 
traction is seen to exist both on the outer and also on the inner wall, and is the first 
indication of the separation of the vestibule from the membranous semicircular canals, 
which are ultimately formed from the terminal portion of the vesicle. At the eighty- 
second hour (fig. 17) the contraction is observed to have become more marked, so as 
partially to subdivide the vestibular sac into two unequal portions ; the lower one, that 
connected with the auditory nerve, is the future vestibule, the upper terminal one 
being developed into the semicircular canals ; the cavity still exists in the interior of 
the vesicle, and the auditory nerve has increased in size. At the close of the fourth 
day (fig. 18) the terminal bulbous portion of the original vestibular sac has become 
very considerably enlarged, and of an oval elongated form ; the contraction originally 
