THE XERYOUS SYSTEM AND THE EYES. 



17 



cular basal membrane, which separates the eye from the surrounding 

 tissues. The nerve becomes connected with this posterior side of 

 the eye. 



While the median eyes of Scorpio thus originate as complicated 

 processes of infolding, the lateral eyes arise very simply as mere 

 depressions of the hypodermis. The simpler structure of the lateral 

 eyes is not sufficient to explain this, so that there must be other 



a. 



fapl¥pt'>j 



<s> 



f; 









^o 



It. 



7)tC«. 



3b. 



h~~ J 



Fig. 11. — Sections through two ontogenetic stages of the lateral eyes of Scorpio. A, earlier, 

 B, later stage; somewhat diagrammatic (after Parker and Laurie). II-V, the optic 

 invaginations; h, hypodermis; in, interneural cells; I, lens; mes, mesodermal tissue; 

 n, optic nerve; pn, perineural cells; r, retina; rli, rhabdom ; sz, retinal cells (terminal cells 

 of the nerve). 



factors which are not yet rightly understood. In relation to this, the 

 transformation of the cephalic pits into the rudiments of the median 

 eyes is remarkable. 



"We have founded our account of the origin of the brain and the 

 eyes on the descriptions of authors who have investigated this 

 subject, but these are for the most part very incomplete, especially 



c 



