138 EMBEYOLOGY OF THE LOWEK VERTEBEATES ch. 



Kerr, 1902). Similar observations have been subsequently made in 

 the case of Amphibians. 



The first obvious step in the specialization of the visual cell is 

 the appearance of a fatty globule in its protoplasm. The end of the 

 cell turned away from the lens now grows out into a projection and 

 pushes back the fine cuticular limiting membrane (external limiting 

 membrane) which has developed over this surface of the retina, into 

 a little pocket. The oil globule which gradually increases in size 

 passes into this pocket (Fig. 76, A, f.g) and lies in it ensheathed in 

 protoplasm. The protoplasm now becomes heaped up into a little 

 conical protuberance (Fig. 76, B, r) which is the rudiment of the 

 rod. At first the limiting membrane is distinct over the surface of 

 the rod but gradually, as the latter assumes a cylindrical shape, its 

 protoplasm takes on a clear structureless appearance throughout : it 

 apparently becomes in fact converted into cuticular material. This 

 cylinder of cuticular material increases in size, assumes a character- 

 istic appearance with alternating discs of dimmer and more trans- 

 parent material as seen in the fixed specimen and the rod is complete 

 (Fig. 76, E and E*). 



The rods complete their development sooner or later according as 

 they are nearer or farther away from the optical axis of the eye and 

 their time of development shows great variation in different individuals. 

 The cones in those Vertebrates in which cones are present are merely 

 specialized rods. 



Lens. — The lens shows in its early stages, in various groups of 

 Vertebrates, departures from the normal condition as described for 

 the Bird, of exactly similar kinds to those seen in the development 

 of the otocyst. In particular, the lens tends to develop out of a solid 

 downgrowth of the deep layer of the ectoderm. This is well seen 

 in Elasmobranchs (Fig. 77, A-E) where a rounded solid lens-rudi- 

 ment is formed by proliferation of the ectoderm, this rudiment 

 becoming isolated and developing a cavity secondarily. It is of 

 interest to notice that even hers a slight dipping down of the external 

 surface into the lens rudiment is apparent for a time (Fig. 77, B). 



In Amphibians, Lung-fishes and Teleostomatous fishes the lens 

 arises in a manner intermediate between what occurs in Elasmo- 

 branchs and what occurs in Sauropsida. In the forms mentioned 

 the lens arises as a downgrowth of the deep layer of the ectoderm 

 (Fig. 77, F-I) and in some cases this downgrowth is simply an 

 invagination of this layer, the only difference from the Sauropsidan 

 condition being that here the opening of the invagination is closed 

 by the superficial layer being continued across it (Fig. 77, J, K). 



As regards the later stages in the development of the lens all that 

 need be said is that it undergoes an enormous increase in size — by 

 absorbing nourishment from its surroundings, for it has no blood- 

 vessels — the cells of the deep wall becoming greatly elongated and 

 taking on a clear glassy appearance, while the superficial wall remains 

 as a layer of cubical epithelial cells over the outer surface of the lens. 



