DEVELOPMENT OF THE EYE. 257 



the primary eye-vesiclo) is thus, transformed into a channel) 

 opening downward. The inverted lower surface attaches 

 itself to the uninverted upper surface of the hollow stalk, so 

 that the hollow space within the stalk, forming the com- 

 munication between the cavity of the twixt-brain and of 

 the i)rimary eye-vesicle, now disappears. The two edges 

 of the channel now grow downward toward each other, 

 enclose the band-like process of the leather-plate, and 

 coalesce beneath it. Thus this process now lies within the 

 axis of the solid secondary optic nerve. It develops into 

 a cord of connective tissue carrying the central blood-vessel 

 of the retina {vasa centralia retince). 



An entirely fibrous covering, the fibrous capsule of the 

 eye, now finally forms round the outside of the secondary 

 eye-vesicle and its stalk (the secondary optic nerve). It 

 originates from the head-plate, from that part of the skin- 

 fibrous layer which immediately encloses the eye-vesicle. 

 This fibrous covering takes the form of a completely-closed 

 globular sac, which surrounds the whole ball of the eye, and 

 on the outer side of this, grows in between the lens and the 

 horn-plate. The globular wall of the capsule soon separates, 

 by fission of the surface, into two distinct membranes. The 

 inner membrane becomes the choroidea, or vascular layer; 

 in front it forms the ciliated crown (corona ciliaris) and 

 the iris. The outer membrane, on the other hand, becomes 

 tlie white enveloping, or protective membrane (sclerotica), 

 and, in front, forms the transparent horny membrane 

 (cornea). The rudiments of all the essential parts of the 

 eye are now formed, and its further development is only in 

 details, in the complex difierentiation and combination of 

 the several parts. 



