HISTOLOGY 



FIG. 223. .4, bit of retina from the eye of a 12 mm. 

 squid, Loligo Pealii; p.lim.m., proximal limiting 

 membrane,- b.m., basal membrane (now situated 

 above the real base at b.) ; vis.c., visual cell bodies ; 

 acc.c., accessory cell bodies; vis.r,, visual rods 

 showing the pigment ascending halfway, about 

 the central axis; nv.f., nerve fibers forming basal 

 layer; mil., mitosis in accessory cell; B, visual 

 rod from a visual cell of a half-grown Octopus 

 Americanum. The animal was out in full tropi- 

 cal sunlight for an hour or two before being 

 thrown into a strong fixative, and the pigment 

 has ascended around the central axis and col- 

 lected as a knob at the distal end. X 920. 



they appear, as in Figure 222, 

 A, resting proximally on a basal 

 membrane. At this time some 

 of them begin to form a layer 

 of visual rods on their distal 

 ends. The rod layer is marked 

 off from the cells by a slight 

 membrane, the limiting mem- 

 brane. 



Figure 222, B, shows the 

 next stage in development, 

 which consists in a proximal 

 migration through the basal 

 membrane of all the rod-bear- 

 ing or visual cells, leaving a 

 single row of their neighbors 

 distal of this boundary (Fig. 

 222, C). These latter cells do 

 not function as supporting ele- 

 ments, but are used to produce 

 and operate two materials: a 

 brown pigment to be used as a 

 protecting pigment, and a fluid 

 which is conducted distally be- 

 tween the rods and forms the 

 outer or distal limiting mem- 

 brane. 



This membrane is of con- 

 siderably greater thickness in 

 some forms than in others, and 

 is exactly homologous with the 

 fluid, vitreous body which was 

 seen in the eye of Strombus. 

 This latter, too, was doubtless 

 formed by the sustentacular 

 cells found in the retina, and 

 was delivered into the eye-sac 

 lumen as the threads we saw 

 passing distally between the 

 visual rods. These cells, which 

 we shall call accessory retinal 

 cells instead of the " limiting 

 cells " of other writers, are in 

 contact proximally with pro- 



