DIFFERENTIATION OF RETINAL ELEMENTS 51 



A separation between the horizontal cells and the nuclei of 

 the visual cells produces the external molecular layer. 

 Finally the transient layer of Chievitz disappears. The 

 amacrine cells and the nuclei of the Mliller fibers (both of 

 inner neuroblastic origin) coalesce with the bipolar and 

 horizontal cells (of the outer neuroblastic layer) and col- 

 lectively form the inner nuclear layer (Figure 33, E). 



The rods and cones appear as protoplasmic extensions in 

 the region of the external limiting membrane. The outer- 

 most boundaries of these cells form what might be termed 

 the embryonic external limiting membrane. The definitive 

 external limiting membrane is formed later by the fibers of 

 Mliller which extend to the outermost boundary of the rod 

 and cone cells, where they form a membrane similar to the 

 internal limiting membrane, but fenestrated. That the 

 external limiting membrane extends out over the inner 

 segments of the rods and cones is unlikely for, as Mann 

 (1928) has pointed out, they appear before the growth of 

 the outer processes of the Mlillerian fibers. Their covering, 

 therefore, must come from the original cell boundaries. 



As regards the origin of the outer segments, Mann (op. cit.), 

 although noncommittal, apparently favors the view that 

 they originate from or can be identified with the cilia which 

 originally lined the cavity of the central nervous system. 

 In this connection she says (p. 94): ^'At 21 mm., although 

 cilia can be seen in the cavity of the fore-brain, the processes 

 from the retina are definitely longer and more widely sepa- 

 rated than before and can be looked on as beginnings of the 

 outer limbs of the rods and cones. They thus would seem to 

 arise by increase in size and reduction in number of pre- 

 existing cilia. Whether or not this is the case is difficult 

 to determine since the structures involved are so delicate 

 and so liable to give rise to artefacts during preparation. 

 It is, however, worthy of note that while an appearance of 

 cilia can be seen continuously on the inner wall of the optic 

 cup, the outer wall cells very definitely lose them soon after 

 the development of the pigment granules. One is therefore 



