704 William Patten 



The axial nerve and the external fibres of the rods in Vertebrates are 

 so strikingly like those found in the Invertebrates, that our supposition 

 that the retina is formed of modified ommatidia receives some confirm- 

 ation. But, if we would compare the rod cells with their axial nerves, 

 to the double retinophorae of the Mollusca, we must carry out the com- 

 parison still further, and suppose that the rod cells are double and con- 

 tain two nuclei, otherwise we cannot explain the presence of the axial 

 nerve in the centre of the cell. Now in the rod cells of the Vertebrates, 

 we actually do find two bodies, one of which is an undoubted nucleus 

 with a nucleolus, and the other a refractive, unnuclear-like body. We 

 find exactly the same bodies in the double retinophorae ofArca, Pecten 

 and Haliotis, and in these cases they are undoubtedly nuclei, by the 

 fusion of whose cells an external nerve fibre came to lie in the centre. 

 Now in the Vertebrates we have the two nuclei , one of which is nucle- 

 olated and the other aborted, and the axial fibre, and we can explain 

 this condition only on the supposition that the rod cells, like the retino- 

 phorae of Mollusca , are formed by the fusion of two cells. They then 

 have all the essential characteristics of retinophorae, and consequently 

 we are justified in supposing that the retina of Vertebrates is formed 

 by the modification of ommatidia. 



There is a striking similarity between the structure of the re- 

 tina in Pecten and that of Vertebrates, due solely to the similarity 

 in conditi ons. Ifwe should shorten the retinophorae oi Pecteii^ so 

 that the nuclei came to lie more in the centre of the retina, above the 

 rods, then we would have the same conditions found in the simple 

 forms of the Vertebrate retina. For instance, beginning at the inner sur- 

 faces of the retinae there would be in both cases : (1 ) a layer of rods; (2) a 

 fibrous layer or membrane (pseudo-membrane of Pecten) separating the 

 rods from the overlying cells; (3) the retinophorae with their nuclei, 

 and the inner layer of gangliouic cells [Pecten) , or Landolt'sche Keule 

 (Vertebrates); (4) a layer of axial nerve fibres {Pecten)^ or basal plexus 

 (Vertebrates) ; (5) the outer layer of ganglionic cells [Pecten, , or ganglionic 

 layer (Vertebrates' , and[6)the nerve fibres of the ganglionic brauch of the 

 optic nerve (Pec^ew) , or the inner layer of optic nerve fibres (Vertebrates). 



In Pecten, the fifth layer, or the outer ganglionic cells, shows the 

 same tendency as in the similar layer of the Vertebrate retina to break 

 up into etages of bipolar and multipolar ganglionic cells. 



I do not cousider that the frequently present, double rod cells of 

 Vertebrates have auything in common with the double retinophorae of 

 Invertebrates. 



