PIGMENT LAYER OF THE RETINA. 2 GO 



ration ; but the fact of their persistence cannot shake the anatomically 

 and physiologically well-grounded fact that the rods and cones 

 represent the terminal organs of the optic-nerve fibres. The same 

 argument also holds against the statements which have recently been 

 made by Manz in a very valuable essay on the eyes of acephalous 

 monstrosities against the nature of the rods and cones.* Their pre- 

 sence in heniicephalic foetuses only proves that the elements of the 

 external layers of the retina can under certain circumstances become 

 developed independently of those of the inner layers, which, if the 

 rods and cones are terminal nerve organs, is in complete accordance 

 with that which is found in other nerves, the peripheric terminal 

 organs of which may be found well developed when the central organs 

 are absent. 



2. THE PIGMENT LAYER OF THE RETINA. 



Although, so far as we know, not directly continuous with 

 the nerve fibres, the layer of pigment cells, ordinarily termed 

 the pigment epithelium of the choroid, still belongs, both phy- 

 siologically and morphologically, to the retina. It is formed 

 during the period of embryonic development from the outer 

 lamina of the primary eye vesicle, which itself proceeds from 

 a protrusion of the foetal brain, and the inner layer of which is 

 metamorphosed into the remaining layers of the retina. At a 

 later period the rods and cones proceeding from the inner 

 layer of the primary eye vesicle grow into the pigment layer, 

 and thus the well-known and very intimate connection between 

 the two is brought about. 



The pigment cells resemble the hexagonal pieces of stone in ' 

 a mosaic, united to form a membrane, the several cells of which 

 however are still capable of being isolated. The external part of 

 each cell adjoining the choroid is poor in pigment or perfectly 

 colourless, and usually contains the spheroidal nucleus, as well 

 as in many animals, the Frog for example, intensely yellow fat 

 globules. The inner portion of the cells contains the peculiar 

 granular colouring matter, and is prolonged in the form of 

 numerous extremely destructible processes between the outer 

 segments of the rods and cones, which last thus come to be 

 imbedded in a pigmented sheath. These sheath-like processes 



* Virchow's Archiv, Band li. 



