454 SLONAKER. [Vol. XIII. 



and many reptiles have the circulation of the pecten. Huschke 

 (26) states that these vessels of the hyaloid membrane and the 

 pecten correspond to the retinal vessels in mammals. They do 

 not, however, penetrate the retina. 



The photographic representations of the fovea and area of 

 the different animals were all taken with the same magnification 

 so that they are directly comparable. In every case the section 

 through the center of the fovea was selected. In some cases 

 the section includes not only the bottom of the fovea, but also 

 some cells of the inner edge of the area beyond. In this case 

 the bottom of the fovea is more or less obscured by these cells. 

 In case the retina does not lie smoothly on the choroid, its 

 position is to be considered abnormal and due either to post 

 mortem changes or to the reaction of the reagents. 



Description. 



Before giving a description of the areas of acute vision in 

 the animals examined, a few words may be necessary regarding 

 the development, form, position, and prevalence of the area and 

 fovea centralis in different vertebrates. 



In the development of a fovea an area centralis is first differ- 

 entiated (27). This stage, according to Chievitz, is present in 

 the human foetus about the sixth month, after which time the 

 fovea begins to appear (29). This takes place by a pitting in 

 of the vitreal surface, or a crowding to the sides, as it were, of 

 the cells in the center of the area. The area differs from the 

 rest of the retina either in thickness or in compactness of cells. 

 In some cases the difference in thickness is easily seen and 

 sharply marked off (PI. XXVII, Figs, i, 3, 6, 7, 8), while 

 in others the increase is very gradual and slight (PI. XXVII, 

 Fig. 5). The increase in thickness is due usually to a greater 

 number of nerve cells, cells of the inner and outer nuclear 

 layers, and greater length of the rods and cones. But an area 

 is not necessarily designated by an increase in thickness of 

 any of these layers because the cells may be more numerous 

 and packed more closely together. This is well illustrated in 

 the nerve-cell layer of the frog's area, which is but a single cell 



