476 



EMBRYOLOGY. 



(c) The Development of the Secondary Optic Cup and the Coats 



of the Eye. 



The optic cup is further metamorphosed at the same time with 



the layer of mesenchyma which en- 

 velops it, and which furnishes the 

 middle and outer tunics of the eye, 

 so that it seems to be desirable to 

 treat of both at the same time. I 

 begin with the stage represented in 

 figures 266 and 269. The optic cup 

 still possesses at this time a broad 

 opening, in which the lens (le) is em- 

 braced. The latter is either separated 

 from the epidermis by only an ex- 

 ceedingly thin sheet of mesenchyma, 

 as in the Mammals (fig. 266), or its 

 anterior face is in immediate contact 

 with the epidermis, as in the Chick 

 (fig. 269). In the beginning, therefore, 

 there is no separate fundament for 

 the cornea between lens and epidermis; 

 moreover, both the anterior chamber 

 of the eye arid the iris are wanting. 



The fundament of the cornea is de- 

 rived from the surrounding mesen- 

 chyma, which, as a richly cellular tissue, 

 envelops the eyeball. In the Chick 

 (fig. 269), as early as the fourth day, 

 it grows in between the epidermis and 

 the front surface of the lens as a thin 

 sheet (bi). At first this sheet is struc- 

 tureless, then numerous mesenchymatic 

 cells migrate into it from the margin 

 and become the corneal corpuscles. 

 These produce the corneal fibres in 

 the same way that embryonic con- 

 nective-tissue cells do the connective- 

 tissue fibres, while the structureless 

 sheet in part goes to form the cement- 

 ing substance between them, and in 

 part is preserved on the anterior and posterior walls as thin layers 



Fig. 269. Section through the an- 

 terior portion of the fundament 

 of the eye in an embryo Chick 

 on the fifth day of incubation, 

 after KESSLEK. 



he, Corneal epithelium ; le, lens-epi- 

 thelium ; h, structureless sheet of 

 the corneal fundament ; bi, em- 

 bryonic connective substance, 

 which envelops the optic cup 

 and, penetrating between lens- 

 epithelium (7e) and corneal epi- 

 thelium (Ae), furnishes the funda- 

 ment of the cornea ; ab, outer, 

 ib, inner layer of the secondary 

 optic cup. 



