240 



HISTOLOGY 



invagination of the surface epithelium which was then cut off and the 

 edges of the outer layer made continuous again. This layer appears 

 in the figure (213, A) as the corneal layer, and is responsible for the 

 formation of the cuticle, which is enlarged at the point above each eye 

 into the cuticular lens. 



The method of development of visual cells on the walls of the invagi- 

 nation is puzzling, and the accounts of various investigators are not 

 entirely satisfactory. The explanation in the case of the accessory eye 

 with its inverted visual cells is evidently correct. The epithelium on 

 the upper or distal wall of the invagination becomes the visual cell-layer. 

 Thus the distal end of the visual cell forms the rhabdome, and the proxi- 



vis, 



FIG. 213. Anterior and posterior eyes of the spider, Epeira diadema. I., cuticular lens; hy., 

 hypodermal layer; rt.c., retinula cells; vis.r., visual rods or rhabdomes; nv.f., nerve fibers. 

 (Alter GRENACHER.) 



mal end (now directed outward) is elongated to constitute the efferent 

 nerve fiber. The other, or inner, wall of the invagination becomes thin 

 and is transformed inlo a tapetum. 



In the principal eye we have a startling exception to the usual rule 

 that the rhabdome is distal and the nerve fiber proximal in an epithe- 

 lium specialized visually. Also we have a completely lost layer to 

 account for if the eye were formed by invagination instead of by delami- 

 nation. This eye, like the other, is described as arising by a posteriorly 

 directed, flat invagination. Its visual epithelium is also developed on 

 the outer wall of this cavity. But the rhabdome appears in the proximal 

 part of the cell, and the nerve processes not only are twisted around so 

 as to leave the distal end of the cell, but they thereby are obliged to pene- 

 trate the lumen (which is here closed to a plane) and to pass through 

 the opposite wall, which is finally lost without leaving a trace in the 

 adult structure. 



