MUSEUM OF COMPARATIVE ZOOLOGY. 91 
The advantages of vision in the new direction may have been due to 
the more favorable relation of the cells to the direction of the newly ad- : 
mitted light as compared with that which came along the original course, 
inasmuch as the latter was nearly perpendicular to the axes of the retinal 
cells (and therefore not favorable, upon Grenacher’s theory, to the per- 
ception of distinct images), whereas the former would be parallel to the 
axes of some of the retinal cells, and therefore competent to furnish (upon 
the development of the lens) a more distinct image. 
Any advantage of this nature would gradually lead to an extension of 
the favorably located portion of the retina, and even to any modification 
of the form of the layer as a whole whereby it should be brought into 
still more favorable optical relations to the newly admitted light. This 
might be accompanied by a gradual regressive modification of parts of the 
retina not so situated as to be capable of profiting by light entering from 
the new direction. In this way the originally symmetrical condition 
would be replaced by conditions more and more unsymmetrical.* 
Thus in time a new lens might be formed and the old one atrophy ; 
one region of the original retina might become converted into a new retina 
with new bacilli at the deep ends of the cells, and the cells of the remaining 
regions sink from their function of percipient elements to that of simple 
pigment-cells. The disappearance of the orzginal bacilli in the persist- 
ently functional area of the original retina might be complete, or only 
partial, 
A strong indication that the anterior median eye in Agelena previously 
existed in the condition of a functional monostichous eye, the deep ends 
of whose retinal cells were directly continuous with the optic-nerve fibres, 
is found in the relation of the optic nerve to the present eye, and espe- 
cially in its relation at different stages of its growth. Without some such 
assumption the peculiar connection of the optic nerve with the retina 
would remain apparently inexplicable ; but upon this assumption the 
conditions appear as a natural consequence of the changes accompanying 
involution. In the earliest stage in which the connection of the optic 
nerve with the retina has been figured, before the appearance of the 
bacilli (Figs. 1, 2), the nerve-fibres emerge from the outer and posterior 
* Grenacher has shown that there is an unsymmetrical condition of the retinal 
cells and their bacilli in the anterior median eyes of Lycosa. (See Grenacher, ’78, 
Taf. III. Fig. 22 A, and text, p. 48.) This must doubtless be regarded as a secondary 
differentiation, — 7. ¢. as evolved after the infolding and from a more symmetrical 
triplostichous condition ; but it is instructive as indicating the possibility of 
regressive changes due to the altered functional requirements imposed on the retina. 
