THE EVIDENCE OF THE ORGANS OF VISION 97 



optic ganglion and of the retinal ganglion ramify to form the mole- 

 cular layers. 



It is evident from Fig. 41 that the retina of Ammoccetes and 

 Petromyzon differs in a striking manner from the typical vertebrate 

 retina. The epithelial part (C) remains the same — viz. the visual 

 rods, the external limiting membrane, and the external nuclear 

 layer; but the cerebral part, the retinal ganglion (A and B), is 

 remarkably different. It is true, it consists in the main of the 

 small-celled mass known as the inner nuclear layer, and of the 

 reticulated tissue or ' neuropil ' known as the inner molecular layer, 

 just as in all other compound retinal eyes; but neither the ganglion 

 cell-layer nor the optic fibre-layer is clearly defined as separate from 

 this molecular layer ; on the contrary, it is matter of dispute as to 

 what cells represent the ganglionic layer of higher vertebrates, and 

 the optic fibres do not form a distinct innermost layer, but pass into 

 the inner molecular layer at its junction with the inner nuclear 

 layer. A comparison of this innermost part of the retina (A, Fig. 

 41), with the corresponding part in Berger's picture of Musca {n.l.o.g., 

 Fig. 38), shows a most striking similarity between the two. In both 

 cases the fibres of the optic nerve (O.n., Fig. 38) which cross at their 

 entrance pass into the ' neuropil ' of this part of the retinal ganglion, 

 and are connected probably (though that is not proved in either 

 case) with the cells of the ganglionic layer. In both cases we find 

 two well-marked parallel rows of cells in this part of the retina, of 

 which one, the innermost, is composed in Ammoccetes of large 

 ganglion-cells, and the other mainly of smaller, deeper staining cells 

 apparently supporting in function. Similarly, also, in Branchipus, as 

 I conclude from my own observations as well as from those of Berger 

 and Claus, the ganglionic layer is composed partly of true ganglion- 

 cells and partly of supporting cells arranged in a distinct layer. This 

 part, then, of the retina of Ammoccetes is remarkably like that of a 

 typical arthropod retina, and forms that part of the retinal ganglion 

 which may be called the ganglion of the optic nerve. 



Next comes the ganglion of the retina (B, Fig. 41) (Parker's first 

 optic ganglion), the cells of which form the small bipolar granule- 

 cells of the inner nuclear layer; granule-cells arranged in rows just 

 as they are shown in Claus' picture of the same layer in the retina 

 of Branchipus (Fig. 40), just as they are found in the cortical layers 

 of the optic ganglion of the pineal eye (ganglion habcnulcr), in the 



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