CH. LIX.] FORMATION OF THE EYE 865 



cerebral vesicle through the canal in its pedicle. It remains con- 

 nected to the diencephalon. It is soon met and invaginated by 

 an ingrowing process from the epiblast of the surface (fig. 662). 

 This process of the epiblast is at first a depression, which ultimately 

 becomes closed in at the edges so as to produce a hollow ball, which 

 is thus completely severed from the epidermis with which it was 

 originally continuous. From this hollow ball the crystalline lens is 

 developed. The way in which this occurs has been described in a 

 previous chapter (see p. 770). By the ingrowth of the lens the 

 anterior wall of the primary optic vesicle is forced back nearly into 

 contact with the posterior, and thus the primary optic vesicle is 

 almost obliterated. The cells in the anterior wall are much longer 

 than those of the posterior wall ; from the former all the layers of 

 the retina are developed, except the layer of pigment cells which is 

 formed from the latter. 



The cup-shaped hollow in which the lens is now lodged is termed 

 the secondary optic vesicle ; its walls grow up all round, leaving, how- 

 ever, a slit below where it meets the lens. This slit is the choroidal 

 fissure. 



The cavity of the secondary optic cup is filled by processes of the 

 neuroglia cells of the retina. Amidst these a process of vascular 

 rnesoblast projects through the choroidal fissure, and by the union of 

 the two the vitreous humour, the lens capsule, and the capsulo-pupillary 

 membrane are formed. In mammals the mesoblastic process projects, 

 not only into the secondary optic vesicle, but also into the pedicle of 

 the primary optic vesicle, invaginating it for some distance from 

 beneath, and thus carrying up the arteria centralis retinae into its 

 permanent position in the centre of the optic nerve. 



This imagination of the optic nerve does not occur in birds, and 

 consequently no arteria centralis retinae exists in them. But they 

 possess an important permanent relic of the original protrusion of 

 the mesoblast through the choroidal fissure, in the pecten, while a 

 remnant of the same fissure sometimes occurs in man under the name 

 coloboma iridis. The cavity of the primary optic vesicle becomes 

 completely obliterated, and the rods and cones get into apposition 

 with the pigment layer of the retina. The inner segments of the rods 

 are the first formed, then the outer. The cavity of its pedicle dis- 

 appears and the solid optic nerve is formed. Meanwhile the cavity in 

 the centre of the primitive lens becomes filled up by the growth of 

 fibres from its posterior wall. The epithelium of the cornea is 

 developed from the epiblast, while the corneal tissue proper is derived 

 from the mesoblast which intervenes between the epiblast and the 

 primitive lens which was originally continuous with it. The sclerotic 

 coat is developed round the eyeball from the general mesoblast in 

 which it is imbedded. The choroid is developed from the mesoblast 



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