ORGANS OF VISION OF THE VERTEBRATA. 407 



character of the Ammocoete eye, evidence derived from its structure must 

 be accepted with caution. 



The vitreous humour. The vitreous humour is derived (except 

 in Cyclostomata) from a vascular ingrowth, which differs considerably 

 in different types, through the choroid slit. Its real nature is very 

 much disputed. According to Kessler's view, it is of the nature of a 

 fluid transudation, but the occasional presence in it of ordinary em- 

 bryonic mesoblast cells, in addition to more numerous blood-corpus- 

 cles, gives it a claim to be regarded as intercellular substance. The 

 number of cells in it is however at best extremely small and in many 

 cases there is no trace of them. In Mammals there appear to be some 

 mesoblast cells invaginated with the lens, which are not improbably 

 employed in the formation of the vessels of the so-called membrana 

 capsulo-pupillaris. In the Ammoccete the vitreous humour originates 

 from a distinct mesoblastic ingrowth, though the cells which give 

 rise to it subsequently disappear. 



The development of the zoiiula of Zinn in Mammalia, which ought to 

 throw some light 011 the nature of the vitreous humour, has not been t'ullv 



O v 



investigated. According to Lieberkiilm (No. 373, p. 43), this structure 

 appears in half-grown embryos of the sheep and calf. 



He says " At the point where the ciliary processes and the ciliary 

 part of the retina are entirely removed, one sees in the meridian bundles 

 of fine fibres, which correspond to the valleys between the ciliary pro- 

 cesses and fill them ; also between these bundles there extend, as a thin 

 layer, similar finely striated masses, and these would have been on the 

 top of the ciliary processes." He further states that these fibres may be 

 traced to the anterior and posterior limb of the lens-capsule, and that 

 amongst them are numerous cells. Kolliker confirms Lieberkiihn's state- 

 ments. There can be little doubt that the fibres of the zonnla are of the 

 nature of connective tissue : they are stated to be elastic. By Lowe they 

 are believed to be developed out of the substance of the vitreous humour, 

 but this does not appear to me to follow from the observations hitherto 

 made. It seeuis quite possible that they ai'ise from mesoblast cells which 

 have grown into the cavity of the vitreous humour, .solely in connection 

 with their production. 



The integral parts of the eye in front of the lens are the cornea, 

 the aqueous humour, and the iris. The development of the latter 

 has already been described, and there remain to be dealt with the 

 cornea, and the cavity containing the aqueous humour. 



The cornea. The cornea is formed by the coalescence of two 

 structures, viz. the epithelium of the cornea and the cornea proper. 

 The former is directly derived from the external epiblast, which 

 covers the eye after the invagination of the lens. The latter is formed 

 in a somewhat remarkable manner, first clearly made out by Kess- 

 ler. 



When the lens is completely separated from the epidermis its outer 

 wall is directly in contact with the external epiblast (future cornea! 

 epithelium). At its edge there is a small ring-shaped space bounded 



