VASCULAR AND FIBROUS TUNICS 



Fig. 44 — Formation of the anterior segment (modeled in optical seaion, based 

 upon the process in primates; scale decreases from a to d). 



a, young embryo (cf. Fig. 40f). Anterior chamber has formed; lids closing over cornea; 

 scleral condensation has appeared. 



b, advanced embryo. Chamber is broader; lids are fused by epithelial plug; cornea has 

 stratified epithelium, mesothelial lining; back wall of chamber still wholly mesodermal, but 

 optic cup margin has put forth a thin outgrowth bearing meridional ridges (ciliary pro- 

 cesses); major circle of iris can be made out. 



c, fetus. Chamber still broader, its margin nearer to major circle; lids still fused but with 

 lash follicles. Meibomian and other glands budding in from ertoderm; Descemet's membrane 

 and canal of Schlemm formed; ciliary muscle and large-vessel layer of chorioid taking shape; 

 hyaloid system has degenerated; continued forward growth of optic cup margin (leaving 

 corona ciliaris behind) has given the iris mesoderm its ectodermal backing (from which the 

 sphinaer is differentiating) but leaves a thin film of mesoderm over the lens — the pupillary 

 membrane, which will soon atrophy. 



d, fetus near term. Chamber will broaden yet more, well past Schlemm's canal; lids re- 

 opened and well differentiated; cornea and anterior sclera fibrous; reaus muscles formed; 

 ciliary muscle fully developed; iris complete — dilatator differentiated, pupillary membrane 

 gone; formerly narrow zone between original optic cup margin and precociously-formed 

 corona now greatly expanded, creating orbiculus ciliaris and leaving old cup margin far 

 behind as the ora terminalis; zonule fibers, growing out from orbiculus, have attached to 

 lens capsule. 



ir- inferior rectus; //- lower lid; ot- ora terminalis; sr- superior rectus; «/- upper lid; zf- 

 zonule fibers. 



