VISUAL TISSUES 



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a small amount of connective tissue between. The corneal fold never 

 entirely closes up in many cephalopods, but leaves a minute water canal. 

 Thus the corneal cavity is filled with sea water, to which is possibly added 

 some secretion of the neighboring cells to keep this water free from para- 

 sites. 



A few histological details should be explained before we proceed to 

 the more important retina. The tissues back of the primary eye-sac 

 become much specialized. They are mostly of various connective-tissue 



FIG. 222. A, B, C. Three stages in the histogenesis of the retina in the cephalopod mol- 

 lusks (A, B, from Sepia; C, from Eledone); b.m., basement membrane, below which the 

 nucleated bodies of the visual cells have migrated in B, C ; p.lim.m., proximal limiting 

 membrane; d.lim.m., distal limiting membrane; vis.c., visual cells; ac.ret.c., accessory reti- 

 nal cells; c.t.c., connective-tissue cells, one of which contains an intra-cellular blood channel; 

 pg., pigment in the distal cytoplasm of the accessory retinal cells; vis.r., visual rods. (After 

 HESSE.) X 600. 



forms, and some develop into smooth muscles to move the eye 1p a small 

 degree. A cartilaginous capsule is formed so that it incloses most of the 

 eye-sac, and is provided with many perforations through which the 

 nerve fibers pass from the retina to the huge optic lobe of the brain. 

 This lobe is usually in very close contact with the eye-sac. The double 

 layer of epithelium which forms the two parts of the lens is an interesting 

 study, for the details of which we have no space ; also the rigid connec- 

 tive-tissue elements on the outside of the eye-sac and iris. 



The retina is naturally the most important tissue. It begins, as in 

 other mollusks, as a columnar epithelium, whose elements lengthen until 



