EYE 



209 



centripetally towards the brain ; centrifugal fibres also arise 

 later. 



In the course of further development, the epiblastic thickening 

 mentioned above, which is often at first hollow, becomes separated 

 from the epiblast, sinks more and more into the interior of the 

 optic vesicle, and is diflferentiated to form the crystalline lens 

 (Fig 167, b). 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 choroid fissure 



V7f 



Am 



AS.^ 



A B 



Fia. 167, A. — Diagram Showing the Mode of Formation of the Primary 

 Optic Vesicles (ABl.) 



VH, fore-brain ; V, V, ventricular cavity of the brain, -wliich communicates freely 

 with the cavities of the primary optic vesicles at ft. 



B. — Semidiaorammatic Figure of the Secondary Optic Vesicle, and of 

 THE Lens becoming separated off from the Epiblast. 



IB, inner layer of the secondary optic vesicle, from which the retina arises ; 

 t, point at which the latter is continuous with the outer layer (AB), from 

 which the pigment epithelium is formed ; H, remains of the cavity of the 

 primary optic vesicle ; L, lens, which arises as a cup-shaped involution of the 

 epiblast (E) ; ', point of involution of epiblast to form the lens ; jl/J/", meso- 

 blastic tissue, which at AP-, Jf ', grows in between the outer epiblast and the 

 lens as the latter becomes separated off, and which gives rise to the cornea as 

 well as to the iris ; C, vitreous chamber of the eye, between the lens and 

 retina, which later becomes filled by the vitreous humour. 



and gives rise to the vitr'eous humour (Fig. 167, b), the bulk 

 of which, as compared with the lens, gradually increases. Blood- 

 vessels (vasa centralia nervi optici, arteria hyaloidea, tunica vasculosa 

 lentis) also extend into the vesicle in the same manner. 



The secondary optic vesicle is thus plentifully supplied with 

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

 a definite vascular and pigmented membrane, the choroid, is formed 

 from the surrounding mesoblast (Fig. 168). 



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

 folds, while more externally it passes in front of the lens to form 



p 



