CHAPTER IV 

 DIFFERENTIATION OF RETINAL ELEMENTS 



The general facts regarding the origin of the optic vesicle 

 from the forebrain and its subsequent invagination to form 

 the optic cup are well known. The story is adequately 

 treated in most textbooks of embryology. The fundamental 

 principles involved are common to all classes of vertebrates. 

 In all forms, the outer layer of the optic cup remains largely 

 undifferentiated and becomes the epithelial pigment layer. 

 The inner (invaginated) layer becomes highly differentiated 

 and specialized to form the elements of the retina proper. 

 The anterior portions of both layers develop into the ecto- 

 dermal part of the ciliary body and iris (pars ciliaris retinae 

 and pars iridica retinae) . 



In the human eye the differentiation of the retina, ac- 

 cording to Mann (1928), can be divided into three arbitrary 

 stages as shown in the accompanying chart, modified slightly 

 from Duke-Elder (1939). 



During the first stage (fourth and fifth week of develop- 

 ment), the wall of the vesicle is like that of the rest of the 

 central nervous system in that it is divisible into two zones: 

 1) the primitive layer of neuro-epithelium containing 8 or 9 

 rows of oval nuclei, and 2) the marginal zone which is devoid 

 of nuclei (Figure 33, A) . The surface which borders the vesicle 

 is covered by many fine cilia like those which persist in the 

 central canal of the spinal cord. These cilia are of interest 

 because of the contention by some embryologists that they 

 eventually form the outer segments of the visual cells (Mann, 

 1928, p. 94; Arey, 1928, p. 890). 



In embryos of 10 mm., differentiation into layers becomes 

 evident. The cells near the inner surface proliferate and 

 migrate outward, a process which occurs most rapidly in the^ 



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