ON THE ORIGIN OF VERTEBEATES. 



239 



nuclei belonging to ganglion cells, while the outermost is 

 composed mainly of distinctly smaller nuclei, which iu 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. 4), 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. 





^Mm 



6 



Fir;. 4. — Retina and optic nerve of Petromyzon (aftei* Miiller and Langerhans). 

 On the left side the Miillerian fibres and pigment epithelium are represented 

 alone. The retina is divided into an epitliclial part 6' (the layer of visual 

 lod cells), and a neurodermal or cerebral part which is formed of A the 

 ganglion of the optic nerve and B the ganglion of the retina. 1, int. limit- 

 ing membrane ; 2, int. molecular layer with its two layers of cells ; 3, layer 

 of optic nerve fibres ; 4, int. nuclear layer ; 5, double row of tangential 

 fulcrum cells ; 6, layer of terminal retinal fibres ; 7, ext. nuclear layer ; 

 8, ext. limiting membrane ; 9, layer of rods ; 10, layer of pigment epitheliiun. 

 D axial cell layer (Axenstrang) in optic nerve. The layer 6 is drawn rather 

 too thick. 



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

 eye of Ammoca?tes and Petromyzon, and traced the development 



^ W. Midler, "Ueberd. Stammes Entwick. d. Sehorgansd. Wirbelth.,"i^(x<(/a^c', 

 C. Ludwig, Leipzig, 1874. 



VOL. XXXV. (N.S. vol. XV.) — JAN. 1901. Q 



