MUSEUM OF COMPARATIVE ZOOLOGY. 67 
I will now describe certain eyes that present exceptions to the con- 
ditions already described. The first will be the eye that shows the 
maximum development; the second, eyes that show the minimum devel- 
opment and the maximum degeneration. 
The specimen that shows the greatest development has already been 
mentioned (page 61) in connection with the choroid and the hyaloid 
membrane, and the reader is referred to the statements there made con- 
cerning these structures. 
The most interesting evidence of unusual development is found in 
the greater differentiation of the layers of the retina; and it is in the 
greater distinctness of the outer nuclear (st. nl. ex.) and outer reticular 
(st. rtl. ew.) layers that the difference chiefly consists (Fig. 21). Whereas 
the former is ordinarily, as in Figure 18, only one or two cells thick, 
(exclusive of the cells st. rel. ex.), in this instance it is three or four cells 
thick ; but more significant than its greater thickness is the fact that 
the deepest cells (nl. ba.) are arranged in a regular layer one cell thick, 
and closely packed. 
What this layer of nuclei represents will be pointed out in the com- 
parative part; it may be said here, however, that it probably does not 
belong to the outer nuclear layer. Although the externa] reticular 
layer even here does not present a well defined boundary either inter- 
nally or externally, — particularly not internally, — the irregular areas 
which have already been mentioned as being destitute of nuclei in the 
average specimens (Fig. 18, st. rtl. ex.) are here considerably more dis- 
tinct (Fig. 21, st. rel. ex.), both as to thickness and frequency of occur- 
rence. In fact, the layer may be regarded as having the characteristic 
structure of the normal eye, excepting that it contains a considerable 
number of scattered nuclei, and is without distinct limitation internally. 
The rods also seem to be better developed in this specimen than in 
others. In many of them what has the appearance of an indistinct 
structureless nucleus may be seen occupying the extremity of the fourth 
of the rod nearest the external limiting membrane (Plate IV. Fig. 22, 
aand b). This one fourth probably represents the inner member of the 
rod. In some cases the nuclear-looking structure terminates on the sidé 
directed toward the distal end of the rod with a well defined straight line, 
but in other cases the whole has a round or elliptical form. This portion 
of the rod takes stain slightly, but itis the only portion that does. The 
substance of the remaining portion is uniformly opaque-glassy in appear- 
ance, excepting that numerous pellucid spots exist in it. . These are con- 
siderably more distinct in some rods than in others, and occasionally 
