AND OF THE MEMBRANOUS LABYRINTH AND AUDITORY NERVE. 
191 
to be thereby confirmed. On examining a chick nine hours after this period, that is, 
at the fifty-fifth hour of incubation (fig. 6), the cavity before so distinctly seen in the 
anterior cerebral cell was much less apparent ; the walls of the cell have also much 
increased in size, not only projecting upwards but laterally, so as partly to cover- 
over the inner portion of the protruded optic vesicle, which at the second day was so 
distinctly seen; the head had also become moi-e curved, which rendered a good ex- 
amination of the eye somewhat difficult. On manipulating the specimen so as to ob- 
tain a view from above downwards, the embi-yo lying on its ventral surface, the outer- 
portion of the protruded vesicle was distinctly seen bounded by a clear defined inner 
border and an outer paler one ; the dark inner one was lost in the dat-k gt-anular mass, 
which by its circular form indicated the r-udimentary eye ; the outer layer was the 
external tegurnentar-y membrane of the embryo. At the sixty-second hour- (Plate IX. 
fig. 8) the optic vesicle is seen to be apparently situated between the anterior and 
middle cerebral cells ; this I believe depends on the great increase of development of 
these cells, and the curved form which the head now assumes, making its apparent 
origin to be ft-om both, as stated by Wagner. On carefully examining it, the exterior- 
dark line was beautifully defined all around, and presented the appearance of an outer- 
covering to the eyeball ; in this appar-ent cavity was seen a pyriform vesicle, the future 
retina, presenting a distinct outline, except at its inner side, where it became constricted 
and of a tubular form, and to all appear-ance was continuous with the cerebral mass ; 
this constricted portion, the tubular end, I suppose to be the optic nerve, the dilated 
portion the retina. Internal to the retina was the circular ci-ystalline lens, with its 
central but indistinct nucleus. The conclusions I have come to, from the observa- 
tions made on the specimens from which the drawings 5, 6 and 8 are taken, are quite 
at variance with those of Huschke. He says that between the second and third 
days the two layers of the retina are formed in the following manner. He describes 
the capsule of the lens as being an inverted portion of the common integument, 
which pushes inwards the dilated end of the optic vesicle, and thus forms a double 
layer; the outer one he describes as Jacob’s membrane, the inner inverted one the 
true retina. In order to ascertain the correctness of this statement, I examined the 
eye at four successive periods between the second and third days, at the forty-eighth, 
fifty-fifth, sixty-second and seventy-second hours. I not only examined them late- 
rally, as they naturally presented themselves on removing them from the egg, but 
both on their dorsal and ventral aspect ; and if the lens had been, as he described, an 
inversion of the integument pressing in the dilated end of the optic vesicle, both of 
the latter positions would have been most favourable for demonstrating it. The lens 
is however formed in quite a different manner ; it is first seen as a rather ill-defined 
granular mass in the cavity of the vesicle itself, containing in its centre a nucleus : 
this is seen on the first half of the third day. On the third it becomes more distinct, 
a well-defined line now bounds its margin ; between the fourth and fifth days, the 
granules become darker and more aggregated towards the centre, leaving a space 
