240 DR WALTER H, GASKELL. 



of the retina; the subsequent paper of KohP does not add 

 anything new, and his drawings are manifestly diagrams, and 

 do not represent the appearances so accurately as Mailer's 

 illustrations. In the accompanying figure, I reproduce in the 

 right side Miiller's picture of the retina of Petromyzon, but 

 have drawn it, as in Langerhans' picture, at the place of entry 

 on the optic nerve. 



From his comparison of this retina with a large number of 

 other vertebrate retinas, he comes to the conclusion that the 

 retina of all vertebrates is divisible into 



A. An ectodermal (epithelial) part consisting of the layer of 



the visual cells, and 



B. A neurodermal (cerebral) part which forms the rest of the 



retina. 



Further, he points out that the neuroderm gives origin through- 

 out the central nervous system to two totally different struc- 

 tures, on the one hand to the true nervous elements, on the 

 other to a system of supporting cells and fibres which cannot 

 be classed as connective tissue, for they do not arise from 

 mesoblast, and are therefore called by him fulcrum cells. In 

 the retina he recognises two distinct groups of such supporting 

 structures — (1) a system of radial fibres with well-marked 

 elongated nuclei, which extend between the two limiting 

 layers, and form at their outer ends a membrana-like expansion 

 which was originally the outer limit of the retina, but becomes 

 afterwards co-terminous with the membrana limitans externa, 

 owing to the piercing through of the external limbs of the rods. 

 This system, which is known by the name of the radial 

 Miillerian fibres, has no connection with (2) the spongioblasts 

 and neurospongium, which form a framework of neuroglia, in 

 which the terminations of the optic ganglion and of the retinal 

 ganglion ramify to form a Gerlach's network (Punktsubstanz). 



Mliller^ then describes the layers of the vertebrate retina as 

 follows :■ — 



A. Ectodermal (epithelial) part of the retina. 



(1) Layer of the visual cells. 



B. Neurodermal (cerebral) part of the retina. 



(2) Layer of the nerve fibre attachments. 



1 Op. cil. - Op, cit., pp. liii-lxxii. 



