THE SENSE-ORGANS 501 



la) and form a gelatinous tissue, the basis of the vitreous 

 humor. From similar mesenchymatous elements added exter- 

 nally is formed the vascular network of the chorioid coat, and 

 outside of this is formed the sclera [sclerotic coat}. The an- 

 terior portion of the chorioid forms the iris and the corre- 

 sponding portion of the sclera forms the cornea. This latter 

 stands out from the lens in front and thus forms an anterior 

 chamber, filled with the aqueous humor, a. colorless lymph, 

 which serves as a refracting medium. The corresponding 

 chamber of the cephalopod eye is perforated by a foramen 

 communicating with the exterior, and through this it is filled 

 with sea water which serves the same purpose. This expe- 



FIG. 136. Development of the optic cup. 



(a) Plastic representation. [After HERTWIG.] (b) Median longitudinal section of 

 <a). (c) Cross section in plane indicated by the line xy. 



dient is comparable with that of the internal ear of selachians, 

 with its direct communication with the exterior through the 

 ductus endolymphaticus. 



The crystalline lens, the formation of which has been alluded 

 to elsewhere, is a product of the ectoderm and appears first as a 

 thickening opposite the optic cup. It soon invaginates and 

 pinches off from its layer of origin, at first as a vesicle with a 

 nearly uniform wall and a central lumen. The posterior wall 

 soon thickens and restricts the lumen more and more until this 

 latter becomes entirely suppressed, while the wall itself, becom- 

 ing lenticular in shape, is covered by the anterior portion as 

 by a cap. This thickening is produced by an extreme elonga- 

 tion of the cells, which remain in the form of a single layer. 



