THE EYE JJ3J 



mesodermic tissue. Tfhich invests the optic cup. gives rise to the sclerotic, 

 cornea, choroid, ciliary body (including the ciliary processes and ciliary 



muscle) and iris. 



Crystalline Lens. — The lens is of ectodermic origin. The surface-ectoderm 

 on the lateral aspect of the head opposite the optic vesicle becomes thickened 

 and depressed to form, as stated, the lens-area. The depressed ectoderm is 

 deepened and converted into a kind of cup. The mouth of the cup becomes 

 constricted, and its hps unite. In this manner the lens-area becomes trans- 

 formed into a closed ectodermic sac, called the lens-vesiele, from which the 

 crystalline lens is difierentiated. The lens-vesicle becomes completely sep. 

 arated from the surface-ectoderm, with which it was originally continuous. 

 It is now received into the optic cup, which has be«i formed in connection 

 with the optic vesicle, its position being just within the mouth of the cup, 

 the circumference of the margin projecting shghtly in advjmce of the vesicle. 



The anterior and posterior walls of the lens-vesicle at this stage consist of 

 several layers oi cylindrical cells, and the vesicle contains a small central 

 cavity. The anterior uall becomes gradually thin, and is ultimately formed 

 of one layer of flattened cells, these cells constituting the anterior epithelium 

 of the adult crystalline lens. The cells of the posterior waU become elongated 

 in a forward direction, obhterating the cavity of the vesicle, and coming into 

 contact with the anterior wall. By this process of cell-elongation the lens- 

 fibres are formed. At the equator of the lens the cells of the anterior and 

 posterior walls merge gradually into one another through the medium of a 

 transitional zone of columnar cells. 



At this stage in its development the crystalline lens consists of (i) an anterior 

 epithelial wall, and (2) a posterior wall composed of elongated cells forming 

 the lens-fibres. 



As development proceeds, additional lens-filM-es are formed by the pro- 

 liferation of cells at the equator of the lens. These fibres are laid down 

 in successive layers, which are arranged concentrically. 



Capsule of the Crystalline Lens. — At an early period in its development the 

 lens becomes invested by a mesodermic capsule, freely supphed with blood- 

 vessels derived from the hyaloid artery and anterior ciliary arteries. This 

 capsule is known as the tunica vasculosa. It persists throughout the period 

 of active growth of the lens, and then undergoes retiogression to form the 

 permanent lens-capsule. The portion of the tunica vasculosa which covers 

 the front part of the lens is called the membrana pupillaris, but this usually 

 disappears prior to birth. It may, however, be present at birth, giving rise 

 to the condition kno^Ti as atresia pupillee. Towards the end of intra-uterine 

 life the timica vasculosa undergoes retrogressicm, and becomes transformed, 

 as stated, into the permanent iens-capsole, which is a transparent, homo- 

 geneous, elastic membrane. » 



According to some authorities the lens-capsule is of mesodermic origin, 

 whilst according to other authorities it is derived from the ceUs of the crystal- 

 line lens. 



Development of the Optie Cap and Optic Stalk. — The optic cup. as stated, 

 is formed by the invagination by the lens-vesicle of the distal or outer wall 

 of the optic vesicle, the invagination also affecting the ventral wall of the 

 optic vesicle and the ventral wall of the part of the optic stalk which is ad- 

 jacent to the optic vesicle, thereby gi^•ing rise to the choroidal fissure. The 

 mouth of the optic cup is directed towards the lateral aspect of the head, 

 and the lens-vesicle hes just within the mouth. The margin of the cup 

 projects shghtly over the lens-vesicle and the circumference of this margin 

 represents tiie outline of the papiL The wall of the cup consists of two layers — 

 namely, inner and outer, the inner representing the distal or outer wall erf 

 the optic vesicle, which has now become invaginated, or pushed inwards. 

 The cup is divisible into two regions — namely, (i ) the ciliary region, adjoiniag 

 the margin of the cup, and (2) the fundus. The line of separation between 

 these two regions corresponds to the ora serrata of the adult eye. 



The ciliary region of the optic cup is associated with the ciliary body (in- 

 cluding the ciliary processes and ciliary muscle) and the iris, which ar« 



