VI.J THE AQUEOUS HUMOUR. 153 



cellular elements have made their way into the cornea, 

 the latter becomes contimious at its edge with the meso- 

 blast 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 more probable that Kolliker'- is right in regarding it 

 as derived from the mesoblast. The grounds for this view are, 

 (1) the fact of its growth inwards from the border of the meso- 

 blast round the edge of the eye, (2) the peculiar relations between 

 it and the corneal corpuscles at a later period. This view would 

 receive stiU further support if a layer of mesoblast between the 

 lens and the epiblast were really present as believed by Lieber- 

 kiihn. 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 Lieberkuhn and 

 other observers, and are not entirely accepted by Kijlliker. It 

 is however especially on the development of these parts in Mam- 

 malia (to be spoken of in the sequel) that the above authors 

 found their objections. 



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 



• L. Kessler, Zur Entwich. d. Auges d. Wirbelthiere. Leipzig, 1874. 

 N. Lieberkuhn, "Beitrage z. Anat. d. embryonalen Auges," Archiv 

 f. Anat. u. Pliys., 1879. KoUiker, Entwick. d. Memchen, etc. Leipzig, 

 1879. 



