THE EVIDENCE OF THE ORGANS OF VISION 



35 



the long cylindrical cells. We see also smaller nuclei in among 

 these larger ones, which look like nuclei of intrusive connective 

 tissue, or may perhaps form a distinct layer of cells, situated between 

 the cells of the anterior wall and the terminations of the nerve 

 end- cells already referred to. 



These elongated cells are in exactly the same position and present 

 the same appearance as the cells of the corneagen layer of any median 



eye. Like the latter they are 



free from pigment and never 

 show with osmic staining any 

 sign of the presence of trans- 

 lucent rhabdite - like bodies, 

 such as are seen in the termi- 

 nation of the retinal cells, and 

 like the latter their nuclei are 

 at the base. The resemblance 

 between this layer and the 

 corneagen cells of any median 

 eye is absolute. Between it 

 and the terminations of the 

 retinal cells there exists some 

 ill-defined material certainly 

 containing cells which may 

 well correspond to Patten's 

 pre-retinal layer of cells. 



Eetina, corneagen, nerve, 

 optic ganglion, all are there, all 

 in their right position, all of 

 the right structure, what more 

 is needed to complete the 

 picture ? 



In order to complete the dioptric apparatus a lens is necessary. 

 Where, then, is the lens in these pineal eyes ? In all the arachnid eyes, 

 whether median or lateral, the lens is a single corneal lens composed 

 of the external cuticle, which is thickened over the corneagen cells. 

 This thickened cuticle is composed of chitin, and is not cellular, 

 but is dead material formed out of the living underlying corneagen 

 cells. Such a lens is in marked contrast to the lens of the lateral 

 vertebrate eye, which is formed by living cells themselves. This 





Fig. 37. — Pineal Eye of Ammoccetes, 

 with its Ganglion Habenulcz. 



