ON THE ORIGIN OF VERTEBRATES. 



257 



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. 9, 

 and the left side of fig. 4. 



2. The retina proper with the retinal ganglion and the optic 

 nerve fibres as already discussed. In this part supporting 

 elements are found just as in any other compound retina of the 



. per 



Fig. 9. — 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 ; p., pigment epithelium ; ji.c.r., pars ciliaris retinoe ; 

 m.f., Mulleriau fibres ; /., lens. 



nature of neuroglia, which are independent of the Miillerian 

 fibres. 



Of these two parts we have already seen that the second is 

 to all intents and purposes a compound retina of a crustacean 

 eye, and seeing that the single-layered epithelial tube is 

 continuous with the single-layered epithelial tube of the central 

 nervous system — i.e., with the cephalic part of the gut of the 

 arthropod ancestor — it follows with certainty that the ancestor 

 of the vertebrates must have possessed two anterior diverticula 

 of the gut, with the wall of which, near the anterior extremity, 

 the compound retina has amalgamated on either side, just as the 

 infracesophageal gangha have amalgamated with the ventral 



