588 TEXT-BOOK OF EMBRYOLOGY. 



The suspensory ligament of the lens, or zonula Zinnii, first appears about the 

 end of the fourth month. By some the fibers of the suspensory ligament 

 are believed to differentiate from the vitreous, by others they are considered as 

 derived from the pars ciliaris retinas. Spaces among the fibers of the ligament 

 enlarge and coalesce to form the canal of Petit. 



The Cornea. — The way in which the mesoderm grows in between the lens 

 vesicle and the surface ectoderm has been described (p. 576). This mesoderm 

 forms a thin almost homogeneous layer containing very few cells. Later that 

 part of the layer which lies against the lens becomes more cellular and vascular, 

 so that it is possible to distinguish between an outer homogeneous non- vascular 

 layer and an inner cellular vascular layer. The former is the anlage of the 

 cornea. Between the two layers vacuoles appear and coalesce to form the 

 anterior chamber of the eye or cavity of the aqueous humor. Subsequent 

 growth of the iris subdivides this chamber into an anterior and a posterior 

 portion. The chamber separates the cornea from the pupillary membrane 

 portion of the tunica vasculosa lentis. Bounding the chamber anteriorly and 

 so forming the posterior layer of the cornea there develops a single layer of 

 flat cells, the so-called "endothelium" of Descemet. Over the surface of the 

 cornea the ectoderm remains and gives rise to a stratified squamous epithelium 

 four to eight cells thick, the anterior corneal epithelium. Just beneath the 

 epithelium a layer of corneal tissue retains its original homogeneous character 

 and forms the anterior elastic membrane or membrane of Bowman. The 

 posterior elastic membrane or membrane of Descemet is usually considered a 

 cuticular derivative of the "endothelium." Throughout the rest of the cornea 

 — substantia propria cornea — cells develop, either by proliferation of the 

 few cells originally present or from cells which grow in from the surrounding 

 cellular mesoderm, and become arranged parallel to the surface as the fixed 

 connective cells of the cornea. 



The Eyelids. — After the lens vesicle becomes separated from the surface 

 ectoderm, the latter folds over above and below to form the first rudiments 

 of the upper and lower eyelids. Each fold consists of a core of mesoderm and 

 a covering of ectoderm. From the mesoderm develop the connective tissue 

 elements of the lids including the tarsal cartilage. From the ectoderm develop 

 the epithelial structures of the lids, the epidermis, the eyelashes and the glands. 

 The edges of the lids gradually approach each other and about the beginning 

 of the third month the epithelium of the upper lid becomes adherent to that 

 of the lower, thus completely shutting in the eyeball. This condition obtains 

 until just before birth. 



The eyelashes develop in the same manner as other hairs (p. 447). 



The Meibomian glands, glands of Moll and the lacrymal glands develop, 

 during the period the lids are adherent, as solid cords of ectoderm which grow 



