HISTOLOGICAL ELEMENTS RETINA NECTURUS 433 
The fovea, when present in the amphibian retina, is small. 
I have not been able to find it in Necturus, and Howard (’08) 
makes no mention of such a ‘spot,’ but Hulke (’67) and Chievitz 
(91)? have seen it in Triton and Salamandra. The frequent 
grouping of 6 to 8 cone cells (figs. 5 to 9) in different parts of the 
retina of Necturus may, perhaps, compensate for the absence 
of a definite fovea. 
Enumerations of nuclei in the outer nuclear layer (tables 8 
and 9) bring out several important features. On the ground that 
each visual cell has its own nucleus there should be in the outer 
nuclear layer a number of nuclei equal to the number of rods 
and cones together, unless elements of another kind should be 
found in either or both the layers compared. The nuclei of the 
outer nuclear layer were found to outnumber the rods and cones 
by about 10,000. Undoubtedly a majority of the nuclei in the 
internal layer together with all those in the external layer should 
be associated with the visual cells; but there is a characteristic 
eroup in the internal layer, having their long axes at right angles 
to the long axes of the nuclei of the visual cells, which appear to 
me to have a different function. They may, perhaps, be hori- 
zontal cells, which Ramon y Cajal (’94) states are represented in 
amphibians by large and small nuclei at two different levels and 
constitute the outermost part of the inner nuclear layer. There 
are, too, in the internal layer a small number of Miiller’s-fiber 
nuclei (fig. 4) whose identity is clearly established by their heavy 
stain and fibers. A fourth kind of nucleus may occasionally 
wander into this layer. Fig. 2 shows how the outer limits of 
the inner nuclear layer may be thrown out of line and individual 
nuclei be protruded far into the outer reticular layer. This 
would seem to favor Bernard’s (00) contention that the nuclei 
of the retinal layers are constantly migrating outward to aid in 
the formation of new elements in the more outwardly lying layers. 
I have been able to observe an apparent migration of this kind 
only in the case cited. 
Conditions in the inner nuclear layer show that it is the most 
variable of all the layers in respect to the number of its elements. 
2 See Slonaker, 1897. 
