132 ANIMAL BIOLOGY. [Parti. 



Meanwhile the cavity of the original optic vesicle (seen in 42, 

 A. and B.) has become completely obliterated, while the stalk 

 has become the solid optic nerve which, in the mammal, is at first 

 somewhat flattened, and is then wrapped round the central 

 artery. Of the two layers of which the optic cup is composed, 

 the posterior (outer) becomes the pigmented layer of the retina, 

 while the rest of the retina is formed out of the anterior (inner) 

 layer. In front of the region of the retina, the limits of which 

 are marked I. r. in 42, D., the two layers become fused together, 

 and with the mesoblastic tissue of the ciliary region ; and beyond 

 this the anterior part of the optic cup forms the posterior portion 

 (or uvea) of the iris. So that the pupil marks the limit of the 

 optic cup, of which only the posterior portion is converted into 

 the retina. 



The remaining parts of the eye are for the most part meso- 

 blastic in origin. The mesoblastic investment of the optic cup 

 differentiates into the choroid internally, and the sclerotic 

 externally. From the mesoblastic choroid the greater part of 

 the iris is derived. The cornea is also for the most part of 

 mesoblastic origin. But the corneal epithelium is derived from 

 the external epiblast which covers the eye after the imagination 

 of the lens, which structure is also, as we have seen, epiblastic 

 in origin. 



In Fig. 42, D., the dark line represents the part derived from 

 the optic cup, which originates from the epiblast of the central 

 nervous system. The lens and corneal epithelium, which are 

 also epiblastic in origin, are cross-shaded. The mesoblastic 

 parts are left unshaded. 



(3.) Hearing. In the frog there is no external ear; but 

 above, and slightly behind the angle of the mouth, the tympanic 

 membrane is stretched over a hard ring, and covered by pig- 

 mented skin. Within this membrane is the tympanum, a funnel- 

 shaped cavity, widening outwards. This communicates with 

 the mouth by the Eustachian passage, and is lined with mucous 

 membrane continuous with that of the mouth. In the tym- 

 panum lies a cartilaginous rod, bony in the middle, the columella 



