THE MOSAIC OF SINGLE AND TWIN CONES IN 
THE RETINA OF FISHES! 
CARL H. EIGENMANN anD GEORGE DANIEL SHAFER. 
THE eyes of fishes have served as the basis of numerous 
papers. A list of these dealing with the eye in its macroscopic 
aspect has been furnished by Ziegenhagen (995) while those 
dealing with its minute structure have been enumerated by 
Krause (86) and Cajal (94). 
During recent years the efforts have been largely in the 
direction of the application of the methods of Golgi and Ehr- 
lich, chiefly to work out the vertical relationships of the various 
layers of the retina. The present paper will be devoted to the 
horizontal relationships of two elements in the retina of fishes, 
t.e., the single and the twin cones. 
. Both elements are well known, and their relative positions 
have also received mention, as will be seen from the following 
summary of the literature. 
The twin cones arise from two cells placed close together; 
their nuclei are usually just within the outer limiting membrane. 
The cone bodies are pressed together so that the faces in con- 
tact are flat. The outer segments are separate. The line 
joining the centers of the two elements of the twin cone may 
be termed the axis of a twin. The single cones vary greatly 
in their relative thickness when compared with the components 
of a twin. Quite frequently they are much thinner than the 
latter, and in some other instances their structure is strikingly 
different. In the eye of Zygonectes, for instance (Eigenmann 
(99), the basal portion of the single cones contains refractive 
granules, increasing in size outward where the series ends ina 
lenticular vacuolated body separating the granular from the 
distal portion. The twins do not possess these granules. In 
! Contributions from the Zoülogical Laboratory of the Indiana University, No. 26. 
L 109 
