DEVELOPMENT OF THE RETIXA. 297 



to birth in Man the development of the rods and cones begins 

 from the outer granule layer has not been accurately ascer- 

 tained. In an embryo of the twenty-fourth week, which I 

 obtained perfectly fresh, I found the membrana limitans interna 

 still quite smooth. Hitter, however, believes he has seen well- 

 developed rods in younger embryoes.* 



In the early stages of its development the retina extends 

 forwards as far as to the border of the lens. In consequence of 

 a difference in the process of development of its several parts, 

 arise the retina proper, its pars ciliaris, and lastly the pigment 

 lying behind the iris, which is covered by only a rudiment of 

 the tissue proceeding from the internal lamina of the primary 

 eye vesicle, which is, it would appear, a variable prolongation of 

 the limitans interna. Inasmuch as during the development of 

 the retina the rudiment of the palpebral fissure of the embryo 

 is indicated by a non-pigmented stria which extends from 

 behind forwards over the whole extent of the retina,f the 

 rudiment of this not unfrequently occasions a persistent 

 absence of pigment as an arrest of development (coloboma), 

 which affects the pigment behind the iris equally with that 

 before the choroid. Coloboma, as Schb'lerJ has already 

 demonstrated, is primarily an arrest of development of the 

 retina, and not of the choroid. Information as to how far the 

 tissues of the latter membrane and of the iris, apart from the 

 pigment epithelium, take part in the frequently occurring 



examination of such specimens. W. Krause macerates the eyes of young 

 Rabbits in bichromate of potash, and finds that the existence of rods and 

 cones can be demonstrated with extraordinary facility, when I am unable 

 to discover them in the fresh state. 



* Grafe's Archiv, Band x., Heft i., p. 75, Heft ii., p. 142. Die Structur 

 der Retina, etc., pp. 32 and 52. 



t See Archiv fur Mikroskop. Anatomie, Band ii., Taf. viii., fig 7. 



J De oculi evolutione. Dissert, inaugural. Mitau, 1849. 



We cannot possibly regard the fovea centralis as a remains of the 

 foetal palpebral fissure, on account of its position, as Hensen has recently 

 pointed out (Archivfur Mikroskop. Anatomie, Band iv., p. 350). On the 

 other hand, the pecten of Birds, and what corresponds to this in Fishes 

 and Reptiles, are situated in the vicinity of the fissure, whilst they 

 originate from the growth of the choroid into it. Schenk, Wiener 

 Stizwngsberichte, 1867. 



