No. 3.] 



ACUTE VISION IN VERTEBRATES. 



473 



The following tabulation, condensed from the foregoing, will 

 show at a glance the prevalence of an area and fovea centralis 

 in the different classes of vertebrates. 



Mammals. 



Mammals as a class are characterized by the absence of a 

 fovea, the primates being the only ones in which it has been 

 found. As a rule an area is present, though in some cases 

 even an area has not been demonstrated. H. Miiller (13) says: 

 " In mammals there is at least an area centralis present which 

 approaches in structure the yellow spot, and is made known by 

 a similar course of the blood-vessels as in man." If such is 

 the case, I have failed in some instances to demonstrate the 

 presence of such an area. 



In some mammals the area is readily seen with the naked 

 eye, but in the majority of those I have been able to examine 

 such is not the case. In many instances vertical and horizontal 

 sections have to be made and subjected to microscopical exam- 

 ination and measurement before a thickening or an arrangement 

 of cells indicating an area is found. In some the very slight 

 thickening is marked also by an increase in thickness of the 

 tapetum. 



I will now proceed to a more detailed description of the 

 area and fovea in the mammals which I have studied. I shall 

 not, however, enter into the histological arrangement of the 

 cells. 



