MUSEUM OF COMPARATIVE ZOOLOGY. 97 
fall upon the sensitive surface more nearly perpendicular to it than they 
would have done without such a curvature. The advantage of this, even 
if an increase in the intensity of the light were the only end achieved, 
is evident ; but, in addition to the increased illumination afforded by this 
part of the tapetum, it is probable that the rays of reflected light would 
take directions more nearly parallel with the axes of the corresponding 
retinal cells (Fig. 27), and that thus conditions favorable for more distinct 
vision — perhaps even for the perception of images — would be realized.* 
Such an advantage once secured at the deep end of the tapetum, it is 
easy to appreciate how an increase in the extent of the curved portion of 
the band would enlarge the more successfully reflecting area, thus en- 
hancing the total effect of the light, and possibly affording a more exten- 
sive (reflected) image. Once begun, this process would not cease until 
it had involved the entire eye. 
This, it seems to me, would be sufficient to explain the curvature 
actually found in the adult eyes, where the retinal cells are all perpen- 
dicular to the tapetum, and would besides afford an explanation of the 
retention of the original bacilli at the (primitively) free ends of the cells. 
It is no longer probable that the iridescent scales of the tapetum are 
referable to the cuticular secretions of the hypodermis. It is more likely 
that the tapetum is formed from cells which grow. from the apex of the 
original retinal involution into the cavity formed by that involution, and 
that they take the form of an outfolding. Whether the tapetal cells, 
phylogenetically considered, originally constituted a distinct portion of the 
hypodermis embracing the area corresponding to the apex of the subse- 
quent involution, it is at present impossible to decide ; but it seems less 
probable than that they should have been gradually differentiated from 
a portion of the retina after the involution (but not the inversion) had 
begun. It may even be imagined that the tapetal scales in some way 
represent the metamorphosed bacillar elements of the cells from which 
they are developed, although I know of no direct evidence of it. Unless 
they are formed from cells which have previously possessed the function 
of retinal elements, their source and the cause of their appearance will be 
still more problematical. 
There is reason to suppose that the course of the optic-nerve fibres 
through the post-tapetal layer is a secondary condition. If—as is prob- 
* That this curvature finally became so great that the light was reflected outward 
through the lens, and thus served to help in the illumination of outside objects, 
does not necessarily interfere with this assumed primitive function of the tapetum. 
VOL. XIII. — NO. 3. 7 
