ORGANS OF VISION. 507 
epiblast. It is true that certain structures usually held to be 
nervous in the compound eye are derived from the surface 
epiblast, but my contention is that the so-called retinal elements 
of the compound eye in Arthropods are not retinal, but re- 
fractive organs. 
In the eye-like organs of Hydrozoa, in which no special tract 
becomes primarily differentiated as a neural epiblast, it is 
possible that certain scattered epiblast cells remain as end 
organs; but it is otherwise in all those animals in which a 
distinct nervous epiblast is differentiated, at an early period 
of development, as a neural epiblast, from which the entire 
nervous system arises. 
In the simple eyes of Arthropods, Grenacher, it is true, 
derives the retinal organs from the epiblast, but he has not 
said a word on their development, and his view rests entirely 
on theoretical grounds; and I shall hereafter show that his 
conclusion is erroneous. Lankester and Bourne, as well as 
Mark, have followed Grenacher, and have arrived at similar 
conclusions, but again without the slightest developmental 
evidence. 
The Eyes of Pecten.—These, as has been already stated, are 
sporadic eyes; they form a double row on the edge of the 
mantle, and are supplied by the circumpallial nerve. Patten 
has investigated their development, and holding as he does the 
view that the retina is derived from the superficial epiblast, 
endeavours to show that it is developed in situ from the super- 
ficial layer of epithelium; but he gives drawings of these eyes 
in an early state which appear to be of great accuracy, and 
which indicate that the retina is developed from neuroblastic 
cells which underlie the superficial epiblast around the extremity 
of a branch of the circumpallial nerve [289, Pl. XXVIIL., Figs. 
3, 4, 11, and 12]. Indeed, as the nerve-filaments enter the 
retinal end organs from their cutaneous surface, it is difficult 
to understand how they can possibly be derived from the 
cutaneous epiblast. 
The Eyes of Nautilus—The structure of the paired cephalic 
eyes of Nautilus is usually relied on as evidence of the origin 
