196 MR. H. GRAY ON THE DEVELOPMENT OF THE RETINA AND OPTIC NERVE, 
the structure of this part, but show the striking analogy which exists between it and 
the retinal expansion. 
At present two opinions exist regarding the evolution of the ear-bulb. It is stated 
by Baer that it arises soon after the appearance of the eye, in the form of a tubular 
prolongation of the brain, which is hollow, and communicates with the cavity of the 
fourth ventricle, its peripheral extremity forming a vesicular dilatation, which is 
gradually separated from the brain. Into this vesicle, which is the analogue of the 
labyrinth, there is protruded inwards a reflection of the integument, which forms 
all the accessory parts of the organ of hearing. Huschke’s account is entirely dif- 
ferent ; he says that the membranous labyrinth does not arise from the brain, but is 
originally a blind sac of the skin with an excretory duct, which gradually contract- 
ing, is at last separated from the common integument and exists as a separate sac 
beneath it. The following observations coincide partly with the description given by 
Baer ; at the same time I shall venture to state some facts in connection with this 
point that have not been (as far as I am acquainted) previously noticed. In the 
embryo chick at the fiftieth hour, soon after the close of the second day, I observed 
that the medulla oblongata was not closed in above, but presented a large open 
shallow cavity, the analogue of the fourth ventricle. At the cephalic extremity it 
communicated with the optic lobes and the anterior cerebral cell by means of a small 
circular orifice. From the central part of the wall of this cavity, and exactly oppo- 
site to the second branchial cleft, the first rudiment of the auditory sac was visible, 
in the form of a small protruded vesicle of a somewhat flattened circular shape ; this 
vesicle was hollow, clear and pellucid, and communicated with the ventricular cavity 
by a small circular orifice. This communication was most distinctly seen on exami- 
ning the embryo from the dorsal aspect, as shown in fig. 13, where the aperture 
leading into the protruded sac is visible on one side, bounded by its well-defined 
margin. In order to satisfy myself of the correctness of this observation, and to test 
the accuracy of Huschke’s statement, I examined numerous embryos every second 
hour, from the thirty-sixth hour to the time when the above appearances were ob- 
served, but in none could I detect a protrusion of the tegumentary layer in that 
situation, and, coinciding as my observations do with those of Baer as regards the 
origin of this membrane, they justify, I think, the conclusions which are above stated. 
At the fifty-sixth hour (fig. 14) the auditory vesicle occupies its usual position; it 
has become however increased in size, and has assumed a pyriform shape, so that 
now it presents a narrow contracted tubular portion, the rudiment of the auditory 
nerve, and a dilated spherical extremity, the auditory sac, or rudimentary vesti- 
bule. This latter portion projects into and becomes encased by the muscular and 
tegumentary layers of the embryo, forming a distinctly marked projection beneath 
the integument; the vesicle itself has become darkly granular and more opake, but 
the cavity in its interior is still distinctly seen communicating with the ventricular 
cavity, through the tubular prolongation of the vesicle (the auditory nerve) ; the 
