No. 3.] VISUAL CELLS INV VERTEBRALIES. 571 
a mechanical function. He does not trace them proximally 
beyond the external limiting membrane. The second is a 
system of parallel spiral fibres running from one end of the 
element to the other (including the nucleus) and always lying 
near the external surface. These he believes to be neurofibrils, 
the conducting elements of these optic organs. The spiral 
fibres described by Ritter (’912; ’91b) and Krause (’92) he 
mentions as possible records of the same feature. That these 
received so little credence (see criticism of Merkel, ’92; Greef, 
:00; Ebner, : 02) he thinks is due largely to the unconvincing 
appearance of the figures. 
Very recently there have appeared publications from Kolmer _ 
(:04), Held (: 04), and Retzius (: 05), describing an appear- 
ance in the visual cells, not to my knowledge reported before, 
namely, a single relatively large peripheral fibre passing from 
diplosomes in the inner segment and over the whole length 
of the outer segment. The demonstration of this fibre was 
obtained by the use of silver-impregnation methods. Like 
results have been reported by Furst (:04) using hematoxylin 
staining on embryonic tissue. 
The investigations upon which the present paper, is based 
were carried on chiefly in the Zoological laboratories of Har- 
vard University at Cambridge, during the years 1902 to 1905. 
A part of the work was done in the laboratories of the United 
States Fish Commission at Woods Holl, during the summers 
of 1902 and 1903. 
Before taking up this special problem I made studies under 
the direction of Professor William A. Locy, upon the devel- 
opment of the Vertebrate retina. In the fall of 1902, at the 
suggestion of Dr. G. H. Parker, I took up as a special prob- 
lem the more limited field of the structure of the visual cells 
in the adult. 
In this work I have received very considerable assistance 
from several persons, for this I owe especial acknowledgment. 
Dr. Parker has given the work his constant supervision and 
