572 HOWARD. [Vor. XIX. 
rendered me every encouragement by helpful suggestions and 
valuable criticism. 
I am indebted to Dr. E. L. Mark for the excellent oppor- 
tunities of the Harvard Zoological laboratories and for help 
in the arrangement of plates, as well as kindly interest at all 
times. 
Thanks are due Professors J. E. Wolff and Charles Palache 
for apparatus and assistance in the use of the same; also to 
Doctors F. B. Mallory and F. H. Verhoeff for suggestions 
in technique. 
In addition to these, there are many to whom I am under 
obligation for kindly assistance. To these I wish here to 
express my thanks. 
ii -OBSERVATIONS: 
A. GENERAL METHODS AND TECHNIQUE. 
I began my study of the visual cells in the frog, Rana 
pipiens Shreber, because of the ease with which this animal 
can be obtained at all seasons, and because of the large size 
of its elements. The importance of the latter consideration 
is obvious and led me finally to study the retine of the large 
salamander, Necturus maculosus Rafinesque (1819), the 
“Mud-puppy” of the St. Lawrence and Mississippi basins. 
The rods in this species proved to have a diameter two and 
a half times that of the rods:in the frog, and larger than 
those of any other vertebrate known to me. This is in keep- 
ing with the well known fact that in Necturus, the histologi- 
cal elements are unusually large, the red blood corpuscle being 
the largest red corpuscle known. As this animal is com- 
monly kept by dealers for supply to zodlogical laboratories, 
it can be obtained readily before ice appears and kept alive 
in laboratory tanks with running or standing water, of a 
depth just sufficient to cover it. The size of the visual ele- 
ments in Necturus led me to use them in preference to those 
of the frog as a basis for my studies. 
