38 



BLIND VERTEBRATES AND THEIR EYES. 



near the eye. In the adult the sclera is a layer of uniform thickness except in the 

 region of the entrance of the optic nerve. It is not usually separated from the 

 adjoining parts of the eye, but in places is retracted a short distance from the 

 choroid coat by the action of reagents. It is for the most part fibrous, with few 

 compressed nuclei, and varies from i8/a to 40 //, in thickness. In the larva a narrow 

 cartilaginous band surrounds all but the ventral wall of the eye. In a specimen 

 35 mm. long the width of the band is about 30 /u., its thickness 16 p. In three adult 

 specimens the sclera of only one had any traces of cartilage. In the right eye of 

 the adult specimen 103 mm. long a cartilage about 36 /a thick, 60 /a wide, and not 

 more than 40 /x long is found on the upper face of the eye. The absence of this 

 cartilage in the adult has probably no connection with the degeneration of the 

 eye. Its presence is probably a larval characteristic which disappears as the gills 

 disappear during the metamorphosis. 



Qpegppp 



Fig. 10. (a) Section of Retina, exclusive of Pigment Cells, of Larva 35 mm. long, (b) Tangential Section through Rods and Cones about on 

 Level with Innermost Extent of Pigment (seen on Right) showing Relative Sizes and Abundance of Rods and Cones, (c) Section of Ret- 

 ina of Larva 48 mm. long, (rf) Section of Retina of Larva 00 mm. long, (c) Tangential Section showing Rods and Cones at about 

 Inner Limit of Pigment (seen on Left). (/) Section of Retina of Adult 106 mm. long, (g) Tangential Section at about Inner Limit of 

 Pigment, (h) Section of Retina of Adult 97 mm. long. 



The average thickness of the cornea is 40 \x. In the adult it is covered by a 

 layer of stratified epithelium, 25 /x in thickness, consisting of three rows of cells. 

 The cells of the inner row are columnar in shape, those of the middle row rounded, 

 and those of the outer row very much flattened and elongated (fig. 9 b). 



In the adult the choroid coat is usually separated from the pigment layer, but 

 adheres closely to the sclera. In general it is thicker at the back part of the eye, 

 and quite decidedly so at the entrance of the optic nerve. The lens is normal. 

 Its size is given in the table on page 40. 



The layers of the retina are well developed in the larva. The retina of the 

 larva differs from that of an Ambly stoma larva in the greater thickness of its gangli- 

 onic layer. This layer is, in the young larva of Typhlotriton, composed of 5 or 

 6 layers of cells. This thickness may in part be an artifact, since the retinae 

 examined are shrunken away from the pigment epithelium and the ganglionic layer 



