THE ORGANS OF SPECIAL SENSE 569 



and differentiation of the cells of the anterior epithelium, in the region where the 

 latter pass over into the lens fibers. (The successive stages in the development 

 of the lens are shown in Fig. 503.) 



The lens capsule becomes differentiated during the third month. It is con- 

 sidered by some as derived from the lens epithelium and of the nature of a 

 cuticular membrane, by others as a product of the surrounding connective 

 tissue. 



By the extension of mesodermic tissue in between the lens and the surface 

 ectoderm, the lens becomes by the end of the sixth week completely surrounded 

 by a layer of vascular connective tissue. This is known as the tunica vasculosa 

 lentis, and receives its blood supply mainly from the hyaloid artery (Fig. 505) 

 which is a foetal continuation of the arteria centralis retina (p. 575). Branches 

 from the hyaloid artery break up into a capillary network which covers both 

 anterior and posterior surfaces of the lens. That part of the tunica vasculosa 

 which covers the anterior surface of the lens is known as the membrana pupillaris. 

 After the earlier and more rapid formation of lens fibers ceases, the hyaloid 

 artery begins (about the seventh month) to undergo regressive changes, and at 

 birth is normally absent. Rarely more or less of the tunica vasculosa fails to 

 degenerate, and if the part w r hich persists is the membrana pupillaris there 

 results a malformation known as congenital atresia of the pupil. 



The Optic Cup. The w^ay in which the optic vesicle becomes transformed 

 into the optic cup has been partially described in considering the development of 

 the lens (p. 566) . The growing lens vesicle appears to push in the outer wall of 

 the optic vesicle while at the same time the edges of the latter are extending 

 around the lens vesicle, until w r hat was originally the outer wall of the optic 

 vesicle lies in apposition with the original inner wall, the cavity of the primary 

 optic vesicle thus becoming completely obliterated (Fig. 504). In this way the 

 optic vesicle is transformed into a two-layered thick-w r alled cup, the cleft be- 

 tween the two layers corresponding to the cavity of the primary vesicle. This 

 cup is at first entirely filled with the developing lens (Fig. 504) . As the cup in- 

 creases in size faster than the lens, the contiguous walls of the cup and lens 

 become separated, the cavity thus formed being the cavity of the vitreous 

 humor (Fig. 505). There seems to be no question but that in Mammals a 

 small amount of mesoderm at first separates the optic evagination from the lens 

 area of the surface ectoderm. This apparently disappears, however, so that 

 the two are in direct contact. It is still an open question whether a thin layer 

 of mesoderm grows in between the edges of the cup and the lens at or just before 

 the beginning of the formation of the vitreous. The lens now no longer fills the 

 optic cup but lies in the mouth of the cup, while at the same time the margin 

 of the cup is extending somewhat over its outer surface, where with the meso- 

 derm it ultimately gives rise to the ciliary body and iris, and forms the 



