548 Royal Society, 



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 cavity,) 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 

 is an offset. The vesicle was hollow, clear and pellucid, and of a 

 flattened circular form. At the fifty-sixth hour it had increased in 

 size and presented a pear-shaped figure ; so that now the narrow 

 contracted tubular portion appeared the first stage in the develop- 

 ment of the auditory nerve ; the dilated portion, the auditory sac or 

 rudimentary vestibule ; and the cavity still existing in its interior 

 and communicating with the ventricular cavity from which it arises, 

 by means of the tubular prolongation, the auditory nerve. The 

 aperture of communication soon becomes smaller and more con- 

 tracted, and this increases as the separation between the auditory 

 vesicle and its parent-cell takes place. At the sixty-fifth hour, be- 

 sides a great increase in the size of the ear-bulb, the auditory nerve 

 has become more distinctly formed, and is quite solidified, so 

 that no communication can now be traced between the ventricular 

 cavity and the vestibular sac. It is in this stage of the development 

 of the auditory apparatus that a great 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 element- 

 ary portions of the auditory apparatus, the auditory nerve and the 

 simple vestibular sac. Such is the simple condition of the organ in 

 the Crustacea and Cephalopod MoUusks. At the seventy-second 

 hour, the vestibular sac has lost its oval form and presents a con- 

 traction around its entire circumference. This 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. 



The minute examination of the development of these structures, 

 of which a consecutive detail is given, leads the author to remark 

 on the almost precise similarity in structure of the membranous 

 labyrinth to the retina in its various stages of development, for it 

 consists like it of a delicate fibrous mesh in the areolae of which is 

 deposited granular matter and numerous nucleated cells, its outer 

 surface being composed of globular-shaped nuclei arranged similar 

 to those covering the outer surface of the retina at an early period 

 of its development. 



From this description a marked similarity may be observed be- 



