ORGANS OF VISION. 50? 



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 [239, PL XXVIII. , Figs. 

 3, 4, n, 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 



