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Mr. J. Cameron. Development of Hie Retina [Feb. 6, 



" On the Development of the Layers of the Eetina in the Chick 

 after the Formation of the Optic Cup." By John Cameron, 

 M.B., Ch.B. (Edin.). Communicated by Professor McIntosh, 

 F.B.S. Beceived February 6 —Bead March 20, 1902. 



(Abstract.) 



The inner wall of the retinal cup in a 4th-day chick has exactly the 

 same structure as the wall of the embryonic cerebral vesicles or spinal 

 cord at the same stage of development. Thus all the structures 

 which His has described in the wall of the embryonic spinal cord can 

 be also recognised in the inner wall of the retinal cup, and may there- 

 fore receive similar names. (1.) A network (the myelospongium), 

 which is produced by the union of the processes of cells called spongio- 

 blasts. The outer and inner extremities of the myelospongium net- 

 work fuse to form the external and internal limiting membranes 

 respectively (the external limiting membrane of the retina corresponds 

 to the internal limiting membrane of the embryonic spinal cord or 

 cerebral vesicle as it is next to the cavity of the original optic 

 vesicle). 



(2.) In the meshes of the myelospongium are two kinds of cells — 

 1st, germinal cells, which are found only under the external limiting 

 membrane ; 2nd, neuroblasts, which are formed from division of the 

 germinal cells and give rise to the ganglion cells and the cells of the 

 inner and outer nuclear layers (the cells of the outer nuclear layer are 

 the youngest cells of the retina). 



Up to the 8th day, the inner wall of the retinal cup grows greatly 

 both in thickness and surface area. On the 8th day of incubation the 

 internal molecular layer appears, and on the 9th day the external 

 molecular layer. These two layers first show themselves in the central 

 point of the retinal cup, and extend forwards in all directions 

 towards the anterior margin of the cup. The outer and inner 

 molecular layers do not extend into the anterior one-fourth of the 

 retinal cup, and this portion shows the simple arrangement seen before 

 the 8th day; but in the posterior three-quarters, the external and 

 internal nuclear layers and the ganglion cell layer are mapped out by 

 the two molecular layers. 



During the 8th day a rearrangement of the myelospongium occurs, 

 and consists in the lateral offshoots of the fibres being absorbed for the 

 most part in the region of the future internal nuclear layer ; but in the 

 region of the internal molecular layer the network becomes finer and 

 much denser, and thus pushes the cells away on either side of it. The 

 internal molecular layer appears before any processes from the 

 ganglion cells or the internal nuclear layer have grown into it, and 



