96 THE ORIGIN OF VERTEBRATES 



molecular layer. Of these two rows of nuclei, he describes the inner- 

 most as composed almost entirely of large nuclei belonging to ganglion 

 cells, while the outermost is composed mainly of distinctly smaller 

 nuclei, which in staining and appearance appear to belong not to 

 nerve-cells but to the true reticular tissue of the molecular layer. 



He also draws special attention to the remarkable layer (5) (5, 

 Fig. 41), which is not found in the retina of the higher vertebrates, 

 the cells of which, in his opinion, are of the nature of ganglion-cells. 



W. Miiller, in 1874, gave a most careful description of the eye 

 of Ammoccetes and Petromyzon, and traced the development of the 

 retina; the subsequent paper of Kohl does not add anything new, 

 and his drawings are manifestly diagrams, and do not represent the 

 appearances so accurately as Miiller's illustrations. In the 

 accompanying figure (Fig. 41) I reproduce on the right-hand side 

 Miiller's picture of the retina of Petromyzon, but have drawn it, as 

 in Langerhans' picture, at the place of entry of 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 neuroclermal (cerebral) part which forms the rest of the 



retina. 

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

 out the central nervous system to two totally different structures, 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 recognizes 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 membrane-like 

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

 becomes afterwards co-terminous with the mcmbrana limitans 

 externa, owing to the piercing through it of the external limbs of the 

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

 Miillerian fibres (shown on the left-hand side of Fig. 41), has no 

 connection with (2) the spongioblasts and neurospongium, which 

 form a framework of neuroglia, in which the terminations of the 



