THE ORGANS OF THE OUTER GERM-LAYER. 471 



tissue cord, with the arteria centralis retinae, which traverses it, is 

 enclosed within the stalk, which is now a quite compact structure. 



Finally the tissue of the intermediate layer, apart from its 

 producing the vitreous body, takes a further active share in the de- 

 velopment of the whole eye, inasmuch as that portion of it which is 

 adjacent to the optic cup is differentiated into the choroid membrane 

 (fig. 266 ch) and the sclerotica of the eye. 



After having thus delineated briefly the source of the most 

 important components of the eye, it will be my purpose in what 

 follows to pursue in detail the development of each part separately. 

 I shall begin with the lens and vitreous body, then pass to the optic 

 cup, and at that point add an account of the formation of the 

 choroid membrane and the sclerotica, as well as the optic nerve ; 

 in a final section I shall treat of the organs that are accessory to the 

 optic cup the eye-lids, the lachrymal glands and their ducts. 



(a) The Development of the Lens. 



When the lens-vesicle has been completely constricted off from th& 

 primitive epidermis (fig. 264-5 Is), it possesses a thick wall, which is 

 composed of two or three layers of epithelial cells, and encloses a 

 cavity that in Birds is partially filled with fluid, but in the case of 

 Mammals by a mass of small cells. The mass of cells is the result 

 of a proliferation of the most superficial flattened sheet of the primitive 

 epidermis ; it is without importance in the further development a 

 transient mass, that soon degenerates and is absorbed when the lens- 

 fibres are developed. (ARNOLD, MIHALKOVICS, GOTTSCHAU, KORAN YI.) 



Externally the epithelial vesicle is sharply limited by a thin 

 membrane, which is afterwards thickened into the capsule of the lens 

 (capsula lentis). There are two opposing views in regard to its 

 development. According to one, the capsule is a cuticular structure, 

 that is to say, a structure secreted by the cells of the lens at their 

 bases ; according to the other view it is the product of a connective- 

 tissue layer, to be described more fully hereafter, enveloping the 

 lens-vesicle. 



In later stages considerable differences arise in the development 

 of the anterior and posterior walls (fig. 266). In the region of the 

 anterior wall the epithelium (le) becomes more and more flattened ; 

 the cylindrical cells are converted into cubical elements, which are 

 preserved throughout life in a single layer and constitute the so-called 

 lens-epithelium in the lens of the adult (fig. 266 le). In the posterior 



