MUSEUM OF COMPARATIVE ZOOLOGY. 97 



fall upon the sensitive surface more nearly perjjendicular 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 exjilanation 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 finall}' became so great that the light was reflected outwaid 

 through the lens, and thus served to help in tlie illumination of outside objects, 

 does not necessarily interfere with this assumed primitive function of the tapetum. 



VOL XIII. NO. 3. 7 



