THE ORGANS OF THE OUTER GERM-LAYER. 481 



alike, become specialised in very different ways, as a result of 

 which there are produced the well-known layers distinguished by 

 MAX SCHULTZE. I shall not go into the details of this histological 

 differentiation, but shall mention some further points of general 

 importance. 



As WILHELM MULLER in his " Stammesentwicklung des Sehorgans 

 der Wirbelthiere " has clearly shown, the development of the 

 originally similar epithelial cells of the retina takes place in all 

 Vertebrates in two chief directions : a part of them become sensory 

 epithelium and the specific structures of the central nervous system 

 ganglionic cells and nerve-fibres ; another part are metamorphosed into 

 supporting and isolating elements into MULLER'S radial fibres and 

 the granular [reticular or molecular] layers, which can be grouped 

 together as epithelial sustentative tissue (fulcrum). Finally, with 

 the descendants of the epithelium are associated connective-tissue 

 elements, which grow from the surrounding connective tissue into 

 the epithelial layer for its better nutrition, in the same manner as 

 in the central nervous system. These ingrowths are branches of the 

 arteria centralis retinae with their extremely thin connective-tissue 

 sheaths. The Lampreys alone form an exception, their retina 

 remaining free from blood-vessels. In all other Vertebrates blood- 

 vessels are present, but they are limited to the inner layers of the 

 retina, leaving the outer granular (Kb'rner) layer and that of the 

 rods and cones free ; the latter have been distinguished as sensory 

 epitheliuni from the remaining portions with their nerve-fibres and 

 ganglionic cells the brain-part of the retina. 



Of all the parts of the retina the layer of rods and cones is the 

 last to be developed. According to the investigations of KOLLIKER, 

 BABUCHIN, MAX SCHULTZE, and W. MULLER, it arises as a product 

 of the outer granular (Korner) layer, which, composed of fine 

 spindle-shaped elements, is held to be, as has been stated, the essential 

 sensory epithelium of the eye. In the Chick the development of the 

 rods and cones can be made out on the tenth day of incubation. 

 MAX SCHULTZE states concerning young Cats and Rabbits, which 

 are born blind, that the fundament of the rods and cones can be 

 distinguished for the first time in the early days after birth ; in 

 other Mammals and in Man, on the contrary, they are formed 

 before birth. 



In all Vertebrates, as long as rods and cones are not present, the 

 inner layer of the optic cup is bounded on the side toward the outer 

 layer by an entirely smooth contour, due to the membrana limitans 



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