58 BULLETIN OF THE 



diately beneath the dermal layer is a layer of comparatively coarse fibres 

 arranged in bundles running more or less nearly parallel with one an- 

 other. Among these bundles are blood-vessels and numerous cells, 

 mostly of the kind that I have regarded as leucocytes. The remaining 

 portion of the tissue in this region is composed of rather fine uniform 

 fibres, containing very few cellular elements. I have said that the eye- 

 enveloping portion of the connective-tissue capsule that is immediately 

 over the lens probably represents the cornea, excepting its epithelial 

 layer. When, however, we bear in inind the method of development of 

 the cornea, — as first clearly made out by Kessler ('77, pp. 83-94), and 

 now understood by all embryologists, excepting in so far as this author 

 believed it to contain, besides its epithelial layer, elements derived 

 from the ectoderm ; and when we remember, further, that in the cornea 

 of the normal adult eye, the substantia propria, together with the mem- 

 brana elastica anterior and the membrane of Descemet, make up its 

 entire thickness, excepting its conjunctival (i. e. epithelial) layer, the 

 interesting question arises whether in such an eye as is represented in 

 Figure 6 the layer crn. should be regarded as representing the whole 

 cornea, or merely the membrane of Descemet. Should the latter inter- 

 pretation be adopted, then it would follow that the tissue intervening 

 between this and the dermal layer would be the substantia propria 

 greatly thickened, and the dermal layer (drm.) would be the membrana 

 elastica anterior of the cornea ; and what I have called embryonal con- 

 nective-tissue cells (Fig. 17, cl. con't.) might then be regarded as corneal 

 corpuscles. However, I hardly think this the right view of the matter, 

 since, as already pointed out, the tissue over the eye is mostly con- 

 tinuous with that of the adjacent regions other than the sclera. Tt 

 is possible that the strands seen at a, Figure 17, give some support to 

 such an interpretation. But whatever view may be taken, it seems to 

 me that we are justified in regarding the conditions here presented as 

 evidence against Kessler's statement that a portion of the cornea, be- 

 sides its epithelial layer, is derived from the ectoderm. This author's 

 account of the development of the cornea in Triton is in substance as 

 follows. 



The first trace of it to appear is a thin layer of hyaline substance 

 on the inner surface of the ectoderm over the eye. This appears at a 

 time when the cavity of the lens vesicle has wholly disappeared, and 

 the retinal layers have begun to be differentiated. (See the author's 

 Figure 60.) This layer is held to be secreted from the ectoderm. The 

 succeeding steps may best be given in the writer's ('77, pp. 89, 90) own 



