30 The American Naturalist. [January, 



molecular layer, thus leaving the rod and cone, and outer 

 nuclear layers without blood-vessels. 4 



Investigations show that not all vertebrates possess foveae, 

 but that each elass has a representative which does. When 

 there is no fovea, a well-defined area centralis is usually pres- 

 ent. However, in some vertebrates, even an area has not been 

 observed. 



The following condensed tabulation will show the frequency 

 of the area and fovea centralis in the eyes which have been 



i 





! 



Areaf w. : 



F _ 



L 



1 



\ 



! |i 



1 



» 



5L. 



'; 



^l 





; ' 





From this tabulation it is readily seen, so far as experimei 

 have gone, that in mammals the presence of a fovea is the ( 

 ception while an area is the rule. The primates are the or 

 mammals in which a fovea has been found. Most of the ma 

 mals examined have a well-defined area which is easily sei 

 but, in some, an area has not been demonstrated. The i 

 rangement of the retinal vessels, however, indicates the pr 

 ence of an area which is free from blood-vessels, and may c 

 respond to the area centralis of other animals. 



' IT. Mttller, '-<- 



5 These results are part.. ilation of J. H. Chievitz in 



er das Yorkommen <ler An-; ( < , i: i r:ili~ r.-tinat- in 



