EETINA 



137 



.Retina. — The fully -developed retina — which it will be re- 

 membered is morphologically a specialized portion of the brain-wall 

 — is an organ of extreme complexity. Its structure even in the 

 adult is by no means completely worked out, and our knowledge of 

 the details of its histogenesis is most imperfect. The most conspicuous 

 feature is the great increase in thickness, the retinal cells becoming 

 slender and columnar in form. Later on the nuclei are seen to become 

 arranged in layers, this being an expression of the fact that the cells 

 are also becoming specialized into layers of different structure and 

 function. The details of development of these are almost completely 

 unknown and there is here an interesting field for investigation. 



The layer of visual or percipient cells lies on the proximal side of 

 the retina, and their rods and cones — the special parts of the visual 

 cells which are believed to have the function of converting light 



AM 



Fig. 76.- 



-Illustrating the development of the rods in Lepidosiren. The upper side of 

 the figures represent the side turned away from the leDS. 



A, B, C, D from stage 35 ; E, fully developed visual cell at stage 38, fixed during exposure to light ; 

 E*, similar element killed in the dark. a.v, annular vacuole ; f.g, fatty globules stained black by 

 osmic acid ; m.Z, external limiting membrane ; n, nucleus of visual cell ; r, rod. 



waves into nerve impulses — are at the ends of the cells which point 

 away from the lens. To reach these rods and cones the light rays 

 have therefore to traverse the whole thickness of the retina. This 

 remarkable arrangement of the retina, precisely the opposite of what 

 we should expect, is one of the characteristic features of Vertebrates. 

 Its morphological significance is however at once made clear by a 

 consideration of the main facts of development of the eye as already 

 outlined. These, in fact, show that what becomes the proximal 

 surface of the retina, i.e. the surface which faces away from the lens, 

 was originally part of the inner surface of the brain rudiment and 

 therefore of the outer surface of the ectoderm before it became 

 involuted to form the brain. 



The visual cells develop therefore on what was originally a part 

 of the outer surface of the body and their rods point in a direction 

 which was originally outwards. The mode in which the rods develop 

 is illustrated by Fig. 76 which is taken from Lepidosiren (Graham 



