NORMAL HISTOLOGY. 



of the rudimentary portions of the optic cup, whose double-layered 

 lip corresponds in position with the pupillary margin. 



The vitreous humor is derived from the mesodermic tissue occu- 

 pying the interior of the optic cup. 



FIG. 392. This tissue appears very early, in 



consequence of the ingrowth of 

 the mesoderm through the cho- 

 roidal fissure ; the early vitreous 

 possesses delicate branched cells 

 as well as numerous blood-vessels, 

 and corresponds to soft embry- 

 onal connective tissue ; later the 

 corpuscles and blood-vessels dis- 

 appear and the mass assumes its 

 characteristic semi - fluid almost 

 structureless condition. The pe- 

 ripheral zone of the vitreous un- 

 dergoes condensation and forms 

 the hyaloid membrane, which 

 in the ciliary region becomes thick- 

 ened and constitutes the suspen- 

 sory ligament of the lens, or the 

 zone of Zinn. 



The eyelids develop as folds of 

 integument above and below the 

 corneal area ; these grow towards 

 one another and finally fuse, all 

 epidermal demarcation for a time 

 disappearing. Shortly b e f o r e 

 birth the centre of the epithelial 

 layer undergoes degeneration and 

 the lids become permanently separated. 



The epithelium of both the tegumentary and conjunctival sur- 

 faces is derived from the ectoderm, as are also such epidermal 

 appendages as the hairs and the glands, the Meibomian glands 

 corresponding to sebaceous follicles in their formation. 



Section through developing eye of thirteen- 

 day rabbit embryo : e, ectoderm ; /, lens, con- 

 sisting of anterior nucleated division repre- 

 senting thin front wall of lens-sac and greatly 

 thickened posterior division, completely filling 

 cavity of sac by elongated fibres whose nuclei 

 present crescentic zone () ; /, posterior pig- 

 mented layer; r, specialized anterior retinal 

 layer ; i, point where layers of optic vesicle be- 

 come continuous ; n, extreme peripheral section 

 of tissue of primitive optic nerve connected wiih 

 vascular tunic (v) occupying posterior surface of 

 lens; m, surrounding mesoderm, which at t 

 grows between lens and retina. 



