HISTOLOGICAL ELEMENTS RETINA NECTURUS 413 
it will be seen at once that, because of the large diameter of the 
visual cells, the number of these elements in a given area will 
be fewer in the amphibia than in any other class of vertebrates. 
The two conditions, viz., large diameter and fewer elements, 
unite to make the amphibian retina especially favorable for a 
study of the numerical relations of the constituent parts. With 
this point in mind I have selected for my investigation the adult 
form of Necturus maculosus (Raf.), the common ‘mud-puppy’ 
of the fresh water streams and lakes of eastern Canada and of 
middle and southern United States (Cope, ’89). The specimens 
were secured by the Zodlogical Department from Venice, Ohio. 
TABLE 2 
Comparative sizes of the outer segments of rods and cones in representatives of the 
different classes of vertebrates (after Miiller ’56) 
DIMENSIONS IN MICRA OF THE OUTER SEGMENTS OF THE 
SPECIES EXAMINED Rods = eal ee Gace: oe Rom 
(pie Me Mies |) lasses Oo Tee 
INTE ee eee Gee 1.5—1.8 | 40.0 —60.0 | 4.0 — 6.0 32.0 — 36.0 
125 (430) (a ee 2nOl—3.10) |) 2030.— 2870 Ol —1on0) 25.0 — 30.0 
Chameleon...... é 1.0—1.3 60.0 — 80.0 
RO Gene aol 6.0 —7.0 | 40.0 — 60.0 5.0 20.0 — 28.0 
IRerchewn asa. of. 2.6 40.0 — 50.0 8.0 — 12.0 
| 
Upon their arrival at the laboratory they were transferred at 
once to a fresh-water cement aquarium in a dimly lighted part 
of the basement of the Museum of Comparative Zoédlogy, where 
they were easily kept in a healthy condition. 
B. Technique 
I have found that the technique usually employed for the 
retina does not give satisfactory results when applied to the optic 
nerve; for this reason I have been obliged to make separate prepa- 
rations for the two structures. 
In order to secure material for a reliable enumeration of the 
retinal elements, a fixation fluid was necessary which would not 
wrinkle the retina, and at the same time could be followed by 
