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. 



m 



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; /, point where layers of optic vesicle be- 

 come continuous ; n, extreme peripheral section 

 of tissue of primitive optic nerve connected with 

 vascular tunic (v) occupying posterior surface of 

 lens; m, surrounding mesoderm, which at / 

 grows between lens and retina. 



