THE OEGANB OP THE OUTER GERM-LAYER. 477 



destitute of cells ; these layers, undergoing chemical metamorphosis, 

 become respectively the membrana elastica anterior and the mem- 

 brane of DESCEMET. 



The internal endothelium of the cornea is developed at an extra- 

 ordinarily early epoch in the Chick. For as soon as the structureless 

 sheet previously mentioned (fig. 259 Ji) has attained a certain thick- 

 ness, mesenchymatic cells proceeding from the margin spread them- 

 selves out on its inner surface as a single-layered thin cell- membrane. 

 With this begins also the formation of the anterior chamber of the eye. 

 For the thin fundament of the cornea, which at first lay in immediate 

 contact with the front surface of the lens, now becomes somewhat 

 elevated from the latter, and separated from it by a fissure-like space 

 filled with fluid (humor aqueus). The fissure is first observable at 

 the margin of the secondary optic cup, and spreads out from this 

 region toward the anterior pole of the lens. The anterior chamber 

 of the eye does not, however, acquire a greater size and its definite 

 form until the development of the iris. 



Two opposing views exist concerning the origin of the structureless sheet 

 which has been described as constituting the first fundament of the cornea in 

 the Chick. According to KESSLEE it is a [product of the secretion of the 

 epidermis, whereas the corneal corpuscles migrate, in from the mesenchyma. 

 In his opinion, therefore, the cornea is composed of two entirely different 

 fundaments. According to KOLLIKER, on the contrary, all its parts are 

 developed out of the mesenchyma, and the homogeneous matrix simply outstrips 

 the cells in its growth and extension. 



In Mammals (fig. 266) the conditions differ somewhat from those 

 of the Chick ; for as soon as the lens- vesicle in Mammals is fully 

 constricted off, it is already enveloped by a thin sheet of mesenchyma 

 (h) with few cells, which separates it from the epidermis. The thin 

 layer is rapidly thickened by the immigration of cells from the 

 vicinity. Then it is separated into two layers (fig. 267), the pupillar 

 membrane (tv) and the fundament of the cornea (h). The former is 

 a thin, very vascular membrane lying on the anterior surface of the 

 lens; its network of blood-vessels communicates on the one hand 

 posteriorly with the vessels of the vitreous body, together with which 

 it constitutes the tunica vasculosa lentis, and on the other anastomoses 

 at the margin of the optic cup with the vascular network of the 

 latter. The fundament of the cornea is first sharply delimited from 

 the pupillary membrane at the time when the anterior chamber of 

 the eye (k) is formed as a narrow fissure, which gradually increase? 

 in extent with the appearance of the iris. 



