THE EYE AND ITS APPENDAGES. 



375 



FIG. 391. 



The posterior wall of the lens-sac plays the active role in the 

 formation of the lens-substance, since the production of the lens- 

 fibres is entirely due to the transformation of its greatly-elongated 

 cells. After the obliteration of the 

 original cavity of the sac has been 

 completely effected by the apposition 

 of the enormously-thickened posterior 

 wall and the anterior lamella, the lens 

 further increases in size by the addition 

 of new fibres at the equator, where the 

 metamorphosis of the epithelial elements 

 into the lens-fibres is continually taking 

 place. 



The anterior and posterior cap- 

 sules of the lens are genetically dis- 

 tinct from the lens-substance, since they 

 are mesoblastic in origin ; for a time 

 they are closely associated with the tran- 

 sient lamellae of vascular mesodermic 

 tissue which invest the surfaces of the 

 lens and constitute the tunicse vas- 

 culosae. The development of the 

 fibrous tunic the sclera and the cor- 

 nea proceeds from the surrounding 



mesoderm, which undergoes condensation immediately around the 

 ectodermic structures representing the retina and the lens. The 

 mesodermic tissue at the sides of the anterior segment grows be- 

 tween the epidermis and the lens, and constitutes a layer of consid- 

 erable thickness ; subsequently this sheet becomes unequally divided 

 by the appearance of a cleft, the primary anterior chamber, into 

 two laminae of unequal thickness ; of these the anterior and thicker 

 becomes the cornea and the posterior and thinner the connective 

 tissue of the iris and the transient vascular tunic of the lens. 



The mesodermic corneal stratum undergoes specialization into 

 the substantia propria, the anterior and posterior limiting mem- 

 branes, and the endothelium, the anterior epithelium alone being 

 ectodermic. 



The choroid and the iris are closely associated in their origin 

 with the mesodermic tract producing the fibrous tunic, the rich 

 vascular net-works characterizing the choroid appearing relatively 

 late. The iris does not grow forward until the anterior chamber 

 begins to form, when it proceeds as a blunt continuation of the 

 choroidal tract ; while the stroma of the iris is contributed by 

 the mesoderm, the pigment-layer is derived from the extension 



Section through developing eye of 

 eleven-and-a-half-day rabbit embryo : 

 B, fore-brain connected with optic vesi- 

 cle (o) nearly effaced by apposition of 

 invaginated anterior segment (r) with 

 posterior wall (f) ; /, lens-sac, com- 

 pletely closed and separated from ecto- 

 derm ; /, tissue within secondary optic 

 cup derived from surrounding meso- 

 derm (*). 



