346 The Nissl’s Substance in the Bird Retina 
vary greatly in size, and a slightly shrunken cell may appear side by side 
with shells showing no signs of shrinkage, seems to prevent this appear- 
ance to be ascribed to cell activity without further consideration. This 
normal variation in the size of the ganglion cells in the same retina, 
as well as the multipolar form of the cells, render comparative measure- 
ments of them both difficult and of doubtful value. 
As regards the changes in the Nissl’s substance of the ganglion 
cells and the bipolar cells during prolonged retinal stimulation these 
results on the bird retina agree perfectly with those of Birch-Hirsch- 
feld on the mammalian retina, with this difference that my preparations 
show a much greater difference between the resting and the stimulated 
retine than do those of Birch-Hirschfeld. But that is to be expected, 
because by my method a more prolonged rest as well as a more pro- 
longed stimulation and consequently more pronounced fatigue were 
assured. In fact, the preparations from the bird retina showing the 
greatest change seem to resemble closely the mammalian retina exposed 
to concentrated electrical light for five minutes (compare fig. 3 b, Plate I 
of this paper with fig. 5b, Plate X, of Birch-Hirschfeld). This lends 
additional support to Birch-Hirschfeld’s view that the changes brought 
about in the cells of the retina by five minutes’ exposure to concentrated 
electrical hght are not pathological but simply due to the greatly aug- 
mented activity of the cells. 
In using the expressions, ‘reduction,’ ‘“ decrease,” ‘ disappear- 
ance,” ete., of the Nissl’s substance I have followed the lead of ‘previous 
investigators. The cell bodies both of the stimulated and the resting 
retine stain intensely blue with Held’s or Nissl’s methylene blue; the 
difference is brought out in the differentiation process, when the cells 
of the stimulated retinz lose the stain more readily, the pretoplasm 
taxing a ditfuse blue stain and the Nissl’s bodies appearing partly fused. 
But yet these bodies are plainly there. The differentiation can be 
carried to a point where the cells of the stimulated retinz have lost the 
blue stain completely, the Nissl’s bodies in the cells of the resting retine 
still retaining some of the stain. This can be accounted for by an 
actual diminution of the substance; that is, a change from the chromo- 
phile to a non-chromophile condition withont any intermediate states, 
in the cells of the stimulated retine; for, provided the differentiation 
fluids diffuse with equal rapidity through the resting and the stimu- 
lated cells, the cells having the less amount of the chromophile material 
will be decolored the sooner. But it could also be explained on the 
assumption that in prolonged activity of the cell the affinity cf a mole- 
cule of the Nissl’s substance for the stain is gradually reduced, to be 
