EYE. 183 



retina is formed, while from the latter the pigment epithelium 



arises. 



In the course of further development, the epiblastic thicken- 

 ing mentioned above becomes separated from the epiblast, sinks 

 more and more into the interior of the optic vesicle, and is 

 differentiated to form the crystalline lens (Fig. 15lB, X). 

 The remaining space within the optic vesicle becomes filled by 

 mesoblastic tissue, which grows in from the ventral side of 

 the secondary optic vesicle through the so-called choroidal 

 fissure, and which gives rise to the vitreous humour (Fig. 

 15 IB, C), the bulk of which, as compared with the lens, gradually 

 increases. Certain vessels (vasa centralia nervi optici, arteria 

 hyaloidea, tunica vasculosa lentis) also extend into the vesicle in 

 the same manner, and are of the greatest importance for the 

 nutrition of the embryonic eye. 



FIG. 152. DIAGRAM OF A HORIZONTAL SECTION THROUGH THE RIGHT HUMAN 



EYE. (Seen from above. ) 



Op, optic nerve ; OS, sheath of optic nerve ; MF, blind spot ; Fo, yellow spot (fovea 

 centralis) ; JSt, retina ; PE, pigment epithelium of the latter ; Ch, choroid, with 

 its lamina fusea (Lf) and vascular layer (GS) ; Sc, sclerotic ; Co, cornea ; Cj, 

 conjunctiva ; MD, membrane of Descemet ; CS, canal of Schlemm (the dotted 

 line should extend further through the sclerotic to the small oval aperture) ; Ir, 

 iris ; Lc, ciliary ligament ; C, ciliary process ; VK, UK, anterior and posterior 

 chamber of the eye ; L, lens ; H, hyaloid membrane ; Z, zone of Zinn ; CP, 

 canal of Petit ; Ov, vitreous humour. 



The secondary optic vesicle is now plentifully supplied with 

 blood-vessels in its interior, and others arise in its periphery, where 

 a definite vascular membrane, the choroid/ is formed from the 

 surrounding mesoblast (Fig. 152, CJi). 



Internally to the lens, the choroid gives rise to the ciliary 

 folds, while more towards the exterior it passes in front of the 

 lens to form the iris (Fig. 152, Ir), which retains in the centre 

 a circular or slit-like aperture, the pupil, through which the rays 



1 I.e. the chorio capillaris and lamina fusca ; the pigment epithelium, as already 

 stated, is formed from the outer wall of the secondary optic vesicle. 



