108 THE ORIGIN OF VERTEBRATES 
serrata, the single epithelial layer vanishes, and the layers of the 
retina take its place. It has long been known, however, that even 
throughout the retina this single epithelial layer still continues, being 
known as the fibres of Miller. This is how the fact is described 
in the last edition of Foster's “Text-book of Physiology,” p. 1308— 
“Stretching radially from the inner to the outer limiting mem- 
brane in all regions of the retina are certain peculiar-shaped bodies 
known as the radial fibres of Miller. Each fibre is the outcome of 
the changes undergone by what was at first a simple columnar 
epithelial cell. The changes 
are, in the main, that the 
columnar form is elongated 
into that of a more or less 
prismatic fibre, the edges of 
which become variously 
branched, and that while the 
nucleus is retained the cell 
substance becomes converted 
into neuro-keratin. And, in- 
deed, at the ora serrata the 
fibres of Miller may be seen 
suddenly to lose their peculiar 
features and to pass into the 
ordinary columnar cells which 
Say the ao 
SINGLE-LAYERED EPITHELIAL TUBE OF form the pars ciliaris retine. 
THE VERTEBRATE EYE AFTER REMOVAL OF It is then absolutely clear 
THE NERVOUS AND RETINAL ELEMENTS. that the essential parts of the 
O.n., axial core of cells in optic nerve; p., eye may be considered as 
pigment epithelium; p.c.r., pars ciliaris 
retine ; m.f., Miillerian fibres; 1., lens. composed of two parts— 
1. A tube or diverticulum 
from the tube of the central nervous system, composed throughout 
of a single layer of epithelium, which forms the supporting axial 
cells in the optic nerve, the pigment epithelium and the Millerian 
fibres of the retina. Such a tube would be represented by the 
accompanying Fig. 44, and the left side of Fig, 41. 
2. The retina proper with the retinal ganglion and the optic 
nerve-fibres as already described. In this part supporting elements 
are found, just as in any other compound retina, of the nature of 
neuroglia, which are independent of the Millerian fibres. 
Fig. 44.—D1asGRaAM REPRESENTING THE 
