CONTRIBUTIONS FROM THE ZOOLOGICAL LABORATORY OF THE MUSEUM OF 
COMPARATIVE ZOOLOGY AT Harvarp CoLiece, E. L. Marx, Director.— 
No. 108. 
THE VISUAL CELLS IN VERTEBRATES, CHIEFLY 
IN NECTURUS MACULOSUS. 
ARTHUR Day HOWARD. 
ke HISTORICAL INTRODUCTION: 
From the earliest recognition of the layer of rods and cones 
as the receptive portion of the optic apparatus, this portion 
of the retina has naturally received considerable attention. 
The history of these researches has been quite fully dealt with 
by a number of writers. Of these I would mention Schultze 
(722), Hoffman (75), Schwalbe (87), Krause (’92), Greef 
(00), aud Elesse (:04 ). 
Treviranus (’36), and after him Gottsche (’36) and Henle 
(739), were among the earliest to declare the layer of rods and 
cones to be the expansion of the optic nerve. ‘The error of 
believing that the “Sehpapillen” were turned toward the light, 
was corrected by Michaelis (°37), who first called attention 
to the fact that the rods formed the outermost layer of the 
retina. This discovery was confirmed by Bidder ('39) and 
Hannover (’40), but both believed that the rods could have 
no nervous function. Brticke (’44) agreed with Bidder and 
with Hannover in considering the rods to be refractive bodies 
for the transmission of light returning by reflection from the 
choroid. 
The classic investigations of Heinrich Miller (’52; °56) and 
Kolliker (’52) helped to bring into general acceptance the 
belief that the rods and cones are the light-receptive organs 
of the retina. One of the grounds upon which Kolliker based 
his conclusion, was the similarity in structure, as he considered 
it, between nerves and rods. He says (p. 328): “Was die 
