ON THE OEIGIN OF VERTEBRATES. 241 



(3) Layer of the tangential fulcrum cells. 



(4) Layer of the ganglion retina?. 



(5) Layer of the spongioblasts. 



(6) Layer of the neurospongium. 



(7) Layer of the ganglion of the optic nerve. 



(8) Layer of the optic nerve fibres. 



The double row of large cells which form in Ammocostes, his 

 layer (3), and Langerhans' layer (5), he does not consider to 

 belong to the nervous part of the neuroderm, but to the system 

 of supporting or fulcrum cells (5, fig. 4). Kohl, on the other hand, 

 agrees with Langerhans, and considers them to be large ganglion 

 cells. None of the three observers have given any satisfactory 

 evidence of the connection of these cells. Miiller speaks of the" 

 nerve fibres as passing between them and not having connection ; 

 Kohl says that the outer row is connected with the cells of the 

 outer nuclear layer, and that the inner row is connected in the 

 adult with the optic nerve fibres, but he has never succeeded in 

 finding any connection between the two rows. 



According to Miiller, the cells of the ganglion retinae — i.e., the 

 inner nuclear layer (4, fig. 4) — are by no means all nerve cells, but 

 a number of them are supporting cells or spongioblasts, especi- 

 ally at the junction of the ganglion with the neurospongium — 

 i.e., the junction of the inner nuclear and inner molecular 

 layers. 



From these figures it is evident that the retina of Ammocoetes 

 and Petromyzon differs in a striking manner from the typical 

 vertebrate retina. The epithelial part (C, fig. 4) 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, fig. 4), 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 

 Punktsubstanz 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 are embedded in the Punktsul)- 



