MUSEUM OF COMPAKATIVK 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. ex.) layers that the difierence chiefly consists (Fig. 21). Whereas 

 the former is ordinarily, as in Figure 18, only one or two cells thick, 

 (exclusive of the cells st. rtl. 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. ha.) 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 external 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. rtl. 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, 

 a and b). This one fourth probably represents the inner member of the 

 rod. In some cases the nuclear-looking structure terminates on the side 

 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 it is 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 



