ORGANS OF VISION OF THE VERTEBRATA. 497 



corpuscles, which have grown in from the sides, thus form a layer 

 which becomes continually thicker, and gives rise to the main 

 substance of the cornea. 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 structureless layer of the cornea is still 

 uncertain. Kessler derives it from the epiblast, but it appears to me more 

 probable that Kolliker is right in regarding it as derived from the meso- 

 blast. The grounds for this view are, (i) 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. This 

 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 priori 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), and 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 have had through Kessler's 

 kindness an opportunity of looking through some of his beautiful prepara- 

 tions, 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 off 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 

 Vertebrata the essential parts above described. The most interesting cases 

 of partial degeneration are those of Myxine and the Ammoccete. The 

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



B. III. 3 2 



