ORGANS OF VISION OF THE VEKTFBKATA. 409 



Whether the increase in the thickness of the layer is due to the 

 immigration of fresh corpuscles, or to the division of those already 

 there, is not clear. After the cellular elements have made their way 

 into the cornea, the latter becomes continuous at its edge with the 

 mesoblast which forms the sclerotic. 



The derivation of the original strnctui'eless layer of the cornea is still 

 uncertain. Kessler derives it from the ei)iblast, but it appears to me more 

 ])robable that Kiilliker is right in regarding it as derived from che meso- 

 blast. The grounds for this view are, (1) the fact of its growth inwards 

 from the border of the mesoblast round the edge of the eye, (2) the peculiar 

 relations between it and the corneal corpuscles at a later period. U'his 

 view would receive still further support if a layer of mesoblast between 

 the lens and the epiblast were really present as believed by Lieberkiihn. 

 It must however be admitted that the objections to Kessler's view of its 

 epiblastic nature are rather a jivioi'i than founded on definite observation. 



The observations of Kessler, which have been mainly followed in the 

 above account, are strongly opposed by Lieberkiihn (No. 374) and Arnold 

 (No. 370), «n<l are not entirely accepted by Kolliker. It is especially on 

 the development of these parts in Mammalia (to be spoken of in the 

 sequel) that the above authors found their objections. I hav'e had througli 

 Kessler's kindness an opportunity of looking through some of his beautiful 

 preparations, and have no hesitation in generally accepting his conclusions, 

 though as mentioned above I cannot agree with all his interpretations. 



The aqueous humour. The cavity for the aqueous humour has 

 its origin in the ring-shaped space round the front of the lens, 

 which, as already mentioned, is bounded by the external skin, the 

 edge of the optic cup, and the lens. By the formation of the cornea 

 this space is shut otf from the external skin, and on the appearance 

 of the epithelioid layer of Descemet's membrane a continuous cavity 

 is developed between the cornea and the lens. This cavity enlarges 

 and receives its final form on the full development of the iris. 



Comparative view of the development of the Vertebrate Eye. 



The organ of vision, when not secondarily aborted, contains in all 

 Vertebi-ata the essential parts above described. The most interesting cases 

 of partial degeneration are those of Myxiiie and the Ammocoete. The 

 development of such aborted eyes has as yet been studied only in the 

 Ammocretei, in which it resembles in most important features that of 

 other Vertebrata. 



Eye of AnnnoCOetes. The optic vesicle arises as an outgrowth of the 

 fore-brain, but the secondary optic cup is remarkable in the young larva for 

 its small size (fig. 291 opv). The thicker (niter wall gives rise to the retina, 

 and the thinner inner waU to the choroid pigment. The lens is formed 

 as an invagination of the single-layered epidermis (fig. 291 I). As develop 

 ment proceeds the parts of the eye giadually enlarge, and the mesoblast 

 around the hinder and dorsal part of the optic cup becomes pigmented. 

 There is at first no cavity for the vitreous humour, but eventually the 



^ The most detailed account is that of W. Miiller (No. 377). 



