CHAP. XLII.] OF THE ORGAN OF HEARING. 601 



About the third day of incubation a fissure, which commences 

 at the border of the lens, is seen in the eye of the chick, which 

 Huschke regards as the consequence of inversion of the retina. 

 In fishes, the cleft running from the centre, towards the anterior 

 border of the retina, exists throughout life. In the turtle, there is 

 a fissure in the nerve but not in the retina. 



Jacob's membrane is not developed before the thirteenth or four- 

 teenth day of incubation. Mr. Gray describes it as forming at 

 this period an exceedingly fine pale granular stratum upon the 

 choroidal surface of the retina. 



The circle of the iris is seen in the anterior part of the choroid 

 at a very early period; but the pupil is occupied by a highly 

 vascular membrane, the membrana pupillaris. This is not attached 

 to the margin of the iris, but to its anterior surface, from which it 

 derives its vessels ; and it is probable that it is reflected over the 

 whole anterior surface of the iris, and possibly lines the anterior 

 chamber of the eye. From the margin of the iris, there extends 

 backwards another vascular membrane, the membrana capsulo- 

 pupillaris, which is united to the border of the capsule of the lens. 

 This membrane forms a closed sac, the anterior part of which is 

 united to the membrana pupillaris; while the posterior portion 

 lines the anterior concave surface of the vitreous body. This is 

 supplied with vessels by the capsular branch of the arteria centralis 

 retinae ; and at the margin of the iris, the vessels of the membrana 

 pupillaris, and those of the membrana capsulo-pupillaris, commu- 

 nicate with those of the iris. 



The eyelids are first developed in the form of a ring, which 

 extends over the surface of the eye; and afterwards the two por- 

 tions which are to be developed into the lids become adherent to 

 each other. They separate agairi, either before birth, as in the 

 human subject or after birth, as in the carnivora and some other 

 classes. 



Organ of Hearing. The ear appears, at a very early period, upon 

 the vesicular protrusion which ultimately becomes the auditory 

 nerve. It communicates with the cavity of the fourth ventricle, 

 and is situated above the second branchial cleft. The first rudi- 

 ments of the auditory vesicle were seen by Mr. Gray about the 

 fiftieth hour of incubation in the chick. Throughout the life of the 

 cyclostomous fishes, the ear retains the condition which it presents 

 at an early period of development in the mammalia. Valentin 

 describes the labyrinth as appearing in the form of a separate body 

 of a somewhat elongated form. The inner extremity forms a turn 



VOL. II. R R 



