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THE ORIGIN OF VERTEBRATES 



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layer of ganglion-cells. In the crustacean, Berger in Squilla, Gren- 



acher in Mysis, and Parker in Astacus, look upon them as supporting 



elements, while Viallanes in 

 Palinurus considers them to be 

 true ganglionic cells. 



Whatever the final interpre- 

 tation of these cells may prove 

 to be, we may, it seems to me, 

 represent an ideal compound 

 retina of the crustacean type by 

 combining the investigations of 

 Berger, Claus, Beichenbach, and 

 Parker in the following figure. 



The comparison of this figure 

 (Fig. 42) with that of the Pe- 

 tromyzon retina (Pig. 41) shows 

 how great is the similarity of 

 the latter with the arthropod 

 type, and how the very points 

 in which it deviates from the 

 recognized vertebrate type are 

 explainable by comparison with 

 that of the arthropod. The 

 most striking difference between 

 the retinas in the two figures is 

 that the layer of terminal nerve 

 fibres (5, Fig. 42), which, after 

 all, are only the elongated termi- 

 nations of the retinal cells be- 

 longing to Parker's neurones of 

 the first order, is very much 

 longer than in Petromyzon or in 

 any vertebrate, for the external 

 molecular layer (G, Fig. 41) 

 (Muller's layer of Nervenan- 



satze) is very short and inconspicuous (in Fig. 41 it is drawn too 



thick). 



Turning from the retina to the fibres of the optic nerve we again 



find a remarkable resemblance, for in Ammoccetes, as pointed out by 



Fig. 42. — Ideal Diageam of the Layers 

 in a Crustacean Eye. 



The retina is divided into an epithelial 

 part, C (the layer of retinular cells and 

 rhabdomes), and a neurodermal or cere- 

 bral part, which is formed of, A, the 

 ganglion of the optic nerve, and, B, the 

 ganglion of the retina. 1, optic nerve 

 fibres which cross at (their entrance into 

 the retina ; 2, int. molecular layer with 

 its two rows of cells ; 3, int. nuclear 

 layer ; 4, Reichenbach's double row of 

 large lightly-staining cells ; 5, layer of 

 terminal retinal fibres ; 6, ext. nuclear 

 layer ; 7, ext. limiting membrane ; 8, 

 layer of crystalline cones ; 9, cornea. 



