352 S. R. DETWILER AND HENRY LAURENS 



The increase in the thickness of the external nuclear layer 

 around the periphery of the fovea is accompanied !)y a corre- 

 sponding increase in the thickness of the external molecular 

 layer. An examination of figure 3 shows that the greatly thick- 

 ened internal nuclear and ganglionic layers of the area become 

 gradually thinned out as the foveal depression is approached, and 

 at its apex are entirely wanting. 



External nuclear layer. The external nuclear layer beyond the 

 foveal limits consists of only one row of nuclei. Typically, these 

 are rounded or slightly oval in shape and lie just beneath the 

 external limiting membrane (fig. 5). Around the periphery of 

 the fovea where this layer is greatly thickened as a result of the 

 tremendous increase in the number of cones the nuclei, instead 

 of being rounded or oval, become considerably elongated and 

 spindle shaped — the long axis of the nuclei being diagonally 

 directed between the external limiting membrane and the exter- 

 nal molecular layer (figs. 3 and 4). The nuclei in this thickened 

 region are not directly applied to the internal surface of the exter- 

 nal limiting membrane but lie deeper in, thus leaving a narrow 

 superficial zone between the external limiting membrane and the 

 nuclei which is occupied by processes connecting the cone nuclei 

 with the inner segment. 



The presence of only one row of nuclei in the external nuclear 

 layer was found in the eyes of the lizards, Sceloporus and Cnemi- 

 dophorus (Detwiler '16). In a pure cone retina, the external 

 nuclear layer is not necessarily composed of only one row. For 

 example, in the turtle (Detwiler op. cit.), where only cones occur, 

 the external nuclear layer consists of two rows of nuclei, Chie- 

 vitz ('89) also found two rows in the eyes of Emys and Lacerta. 

 He considers the row just internal to the external limiting mem- 

 brane to be the nuclei of the cones, and the second row to be the 

 nuclei of supporting cells. In the Chameleon eye, Rochon- 

 Duvigneaud ('17) figures but one row of nuclei, while in the eye 

 of the Gecko (which possesses rods only) he shows the external 

 nuclear layer to be several cell layers deep. In the eye of Alli- 

 gator mississippiensis (Laurens and Detwiler '20), the external 

 nuclear layer is double — the outer row being the nuclei of rods, 

 the inner of cones. In the crocodile, Heinemann (77) described 



