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. Miller, 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 Miller'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 neurodermal (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 membrana 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 
