DEVELOPMENT OF THE SENSE ORGANS 383 



the lens (Fig. 379). This cavity is the anterior chamber of the eye and sepa- 

 rates the anlage of the cornea from the lens capsule. 



An inner layer of mesenchyme, between the anlage of the sclerotic and 

 the pigment layer of the retina, becomes highly vascular during the sixth 

 month. Its cells become stellate in form and pigmented, so that the tissue 

 is loose and reticulate. This vascular tissue constitutes the chorioid layer, 

 in which course the chief vessels of the eye. The chorioid layer corresponds 

 to the pia mater of the brain. Distal to the ora serrata of the retinal layer, 

 the chorioid is differentiated into: (i) the vascular folds of the ciliary 

 bodies; (2) the smooth fibers of the ciliary muscle; (3) the stroma of the 

 iris. The proximal pigmented layers of the iris are derived. from the pars 

 iridica retinae and from a corresponding zone of the pigment layer. Of 

 these, the pigment layer cells give rise to the sphincter and dilator muscles 

 of the iris. These smooth muscle fibers are thus of ectodermal origin. 



The Eyelids appear as folds of the integument in 20 mm. embryos. 

 The lids come together and the epidermis at their edges is fused in 33 mm. 

 embryos (Fig. 379). Later, when the epidermal cells are cornified, sepa- 

 ration of the eyelids takes place. A third, rudimentary eyelid, corespond- 

 ing to the functional nictitating membrane of lower vertebrates, forms the 

 plica semilunaris. The epidermis of the eyelids forms a continuous layer 

 on the inner surfaces, known as the conjunctiva, which in turn is continuous 

 with the anterior epithelium of the cornea. 



The Eyelashes, or cilia, develop like ordinary hairs and are provided 

 with small sebaceous glands. In the tarsus, or dense connective-tissue 

 layer of the eyelids, which lies close to the conjunctival epithelium, there 

 are developed about 30 tarsal (Meibomian) glands. These arise as in- 

 growths of the epithelium at the edges of the eyelids, while the latter are 

 still fused. 



The Lacrimal Glands appear in embryos of about 25 mm., according 

 to Keibel and Elze. They arise as five or six ingrowths of the conjunctiva, 

 dorsally and near the external angle of the eye. The anlages are at first 

 knob-like, but rapidly lengthen into solid epithelial cords. They begin to 

 branch in 30 mm. embryos. At stages between 50 and 60 mm. (C R), 

 additional anlages appear which also branch. 



In 38 mm. (C R) embryos, a septum begins to partition the gland into orbital and pal- 

 pebral portions. This septum is complete at 60 mm. (C R), the five or six anlages first de- 

 veloped constituting the peripheral orbital part. Lumina appear in the glandular cords in 

 fetuses of 50 mm. (C R) by the degeneration of the central cells. Accessory lacrimal glands 

 appear a month before term. The lacrimal gland is not fully differentiated at birth, being 

 only one-third the size of the adult gland. In old age marked degeneration occurs. 



The N ' aso-lacrimal Duct arises in 12 mm. embryos as a ridge-like 

 thickening of the epithelial lining of the naso-lacrimal groove (Fig. 149); 



