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VISION 



stations of the retina, although such direct anatomical cell-to-cell connection 

 may be an oversimplification. Two basic types of vertebrate bipolar cells, 

 each associated with the two photoreceptor types, were described long ago 

 (Schiefferdecker 1886) and their existence was confirmed in sharks 

 (Neumayer 1897). 



The only modem work on bipolar neurons of sharks is that of Witkovsky 

 and Stell (1971, 1973). Retinal bipolars of Mustelus were examined under 

 the light microscope after Golgi impregnation or vital staining (methylene 

 blue). These were found to conform to the general vertebrate plan: that is, 

 axons were either fine or displayed bulbous terminal expansions (Figure 13). 

 Several other factors were considered in classifying the bipolar cells of 

 Mustelus. Thus, five repeatedly observable types were reported. On two of 

 the subtypes there was a clublike process extending into the receptor layer. 

 These processes were first described by Landolt (1871) in the amphibian 



Landolt's 

 club 



Figure 13 The organization of innerplexiform layer in the 

 retina of Mustelus canis. The five classes of bipolar cells and 

 three of the several types of ganglion cells are shown, aj = 

 monostratified bipolar cell with cylindrical Landolt's club 

 terminating in distal third of IPL. a 2 = multistratified bipolar 

 with filamentous Landolt's club, b = monostratified bipolar 

 terminating in middle third of IPL; no Landolt's club, c± = 

 monostratified bipolar terminating in proximal third of IPL. 

 C2 = same except multistratified; no Landolt's club. D, I, O, = 

 displaced, intermediate, and ordinary ganglion cells. EPL = 

 external plexiform layer. INL = inner nuclear layer. IPL = 

 internal plexiform layer. GCL = ganglion cell layer. (Redrawn 

 from Witkovsky and Stell (1973). Reproduced by kind per- 

 mission of the authors and the Journal of Comparative Neu- 

 rology © 1973 Wistar Press.) 



