456 SLONAKER. [Vol. XIII. 



fibre. In the periphery of the retina each rod and cone nucleus 

 lies vertically over the rod or cone to which it belongs, and the 

 cone nucleus is in the base of the cone. But in the region of 

 the fovea these nuclei are crowded toward the periphery and lie 

 some distance from the rods and cones to which they belong. 

 This causes a diagonal appearance of these fibres in a cross 

 section of the retina. The cone nuclei here lie three or four 

 deep. The rods gradually decrease in relative number toward 

 the area or fovea, and in the center of the fovea they are wanting, 

 cones only being present. Borysiekiewicz (30) states, however: 

 " Within the fovea centralis all distinguishing characteristics be- 

 tween the rods and cones are lacking; the so-called cones of the 

 fovea entirely resemble the rods in the periphery of the retina, 

 it is therefore correct to speak, not of the cones, but of the rods 

 of the fovea." Most investigators do not uphold his view. 



The length of the rods and cones in the fovea and area 

 varies considerably in different animals. They may be longer 

 than in other parts of the retina, as in the crow (PI. XXIX, 

 Fig. 52), or shorter, as in the ring-neck plover (PI. XXIX, Fig. 

 47). This difference in length of rods and cones is shown by 

 a greater or less thickness of the pigment layer. Dimmer (29) 

 thinks in the hiiman macula the rod and cone layer is of about 

 the same thickness as elsewhere. 



If the front half and vitreous humor is removed from the 

 hardened eye, in many animals the area is readily seen as a 

 whiter region which has various shapes and positions. It is 

 never sharply marked off from the surrounding retina, but 

 gradually blends with it. Its form may be circular, oval, or 

 band-like. In the latter case it may also contain a circular 

 .area, as in the ring-neck plover, goose, and tern (PL XXVII, 

 Figs. 8, 12). The round area may be situated on the nasal side 

 of the nerve entrance, in which case it is designated area 

 nasalis ; or it may be on the temporal side, and designated 

 ai-ea temporalis. Usually at least one of these different kinds 

 of areas is present, and all three may be present as in the tern. 

 In many eyes, however, when examined in this macroscopical 

 way, no area is visible. It is not until sections are made and 

 subjected to careful microscopical examination and measure- 



