478 



EMBRYOLOGY. 



r pi H 



k 1. 2. 8. Ip ech D 



lie 



Fig. 270. Section through the 

 margin of the optic cup of 

 an embryo Turdus musicus, 

 after KESSLEB. 



y, Retina ; pi, pigmented epithe- 

 lium of the retina (outer 

 lamella of the optic cup) ; 

 li, connective-tissue envelope 

 of the optic cup (choroidta 

 and sclera) ; * ora seirata 

 (boundary between the mar- 

 ginal zone and the fundus of 

 the optic cup) ; ck, ciliary 

 body; 1, 2, 3, iris; 1 and 2, 

 inner and outer lamellae of 

 the pars iridis retinas ; 3, con- 

 rective-tissne plate of the 

 iris ; Ip, ligaruentum pecti- 

 natum iridis ; sch, canal of 

 SCIU.KMM ; D, DESCEMET'S 

 membrane ; h, cornea ; he, 

 corneal epithelium. 



During these processes the condition of 

 the optic cup itself has also changed. Its 

 outer and inner lamellae continually be- 

 come more and more unlike. The former 

 (figs. 266, 267 pi) remains thin and com- 

 posed of a single layer of cubical epi- 

 thelial cells. Black pigment granules are 

 deposited in this in increasing abundance, 

 until finally the whole lamella appears 

 upon sections as a black streak. The 

 inner layer (r), on the contrary, remains 

 entirely free from pigment, with the ex- 

 ception of a part of the marginal zone ; 

 the cells, as in the wall of the brain- 

 vesicles, become elongated and spindle- 

 shaped, and lie in many superposed layers. 



Moreover the bottom of the cup and 

 its rim assume different conditions, and 

 hasten to fulfil different destinies; the 

 former is converted into the retina, the 

 latter is principally concerned in the 

 production of the ciliary body and the 

 iris. 



The edge of the cup (fig. 267 rz, fig. 270*, 

 and fig. 271) becomes very much reduced 

 in thickness by the cells of its inner layer 

 arranging themselves in a single sheet, 

 remaining for a time cylindrical, and then 

 assuming a cubical form. But with its 

 reduction in thickness there goes hand 

 in hand an increase in its superficial 

 extent. Consequently the margin of the 

 optic cup now grows into the anterior 

 chamber of the eye between cornea and 

 the anterior surface of the lens, until it 

 has nearly reached the middle of the 

 latter. Then it at last bounds only a 

 small orifice which leads into the cavity 

 of the optic cup the pupil. The pigment 

 layer of the iris is derived from the mar- 

 ginal region of the cup, as KESSLER first 



