No. 2.] HISTOGENESIS OF THE RETINA. 431 



canal, and a later differentiation after the same plan as in the 

 embryo.^ The structure of the ora serrata of the human 

 retina also indicates that this must be the growing point As 

 we follow the retina into this point we find that the granular 

 layers disappear — first the outer and then the inner. The two 

 nuclear layers then run together, and finally the rods and cones 

 disappear.^ In this region there is a decided increase of 

 Miiller's fibers or spongioblasts. By passing from the ora 

 serrata to the retina the layers appear in the order of their 

 development in the embryo, This explains to us why the 

 structures of the retina "flow together" as the ciliary border 

 is approached. 



I do not think it probable that the cells of the outer nuclear 

 are liable to multiply under any normal conditions. According 

 to Boquis ^ and to Falchi^ direct injury of the retina causes 

 multiplication of the cells of that part. The latter author states 

 that the ganglion cells as well as the cells of the inner and 

 outer nuclear layers may divide. I do not think that the papers 

 of the above authors can bear rigid cross examination, and it 

 is probable that most of the cell divisions were seen in the 

 inflammatory tissue, or at the highest in the spongioblasts.^ 

 Under certain circumstances, however, it seems as if the mature 

 retina may begin and grow anew. This is by no means an 

 exception for the retina alone, for in all pathological new- 

 formations the same may be true. Flexner^ has described a 

 tumor arising from the outer nuclear layer, and this whole 

 new-formation (neuroepithelioma as he calls it) is composed of 

 round cells and circular groups of rods and cones. We must 

 interpret the above case as a multiplication of cells which are 

 already highly differentiated. All of the cells of the outer 



^ Caporaso, Ziegler's Beitrage, Ed. V. 



2 Schwalbe, Anat. d. Sinnesorgane, 1887. 



3 Riforma Medica, IV. 



* Riforma Medica, IV, and Ziegler's Beitrage, Bd. V. 



s Martin states that the spongioblasts of the embryonic cat's retina multiply. 

 Recently Kerestszeghy and Hannss (Ziegler's Beitrage) Bd. XII, have shown that 

 when the dog's spinal cord is injured the glia cells increase to a slight extent, and 

 that the ganglion cells do not multiply. 



s Flexner, Johns Hopkins Hospital Bulletin, 1891. 



