io8 



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 Miiller. 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 Miiller. 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 Miiller may be seen 

 suddenly to lose their peculiar 

 features and to pass into the 

 ordinary columnar cells which 

 form the pars ciliaris retime." 

 It is then absolutely clear 

 that the essential parts of the 

 eye may be considered as 

 composed of two parts — 



. p.c r 



- P 



- aa.t 



Fig. 44. — Diagram representing the 

 Single-layered Epithelial Tube of 

 the Vertebrate Eye after removal of 

 the Nervous and Retinal Elements. 



O.n., axial core of cells in optic nerve; 2 } -> 

 pigment epithelium; p.c.r., pars ciliaris 

 retina ; m.f., Miillerian fibres; I., lens. 



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 Miillerian 

 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 Miillerian fibres. 



