332 



STUDY OF PIG EMBRYOS. 



ment layer in figure 223 is probably artificial, the result of shrinkage during the 

 preservation of the specimen. At its outer edge the retina, Ret, suddenly thins out 

 and passes over into the external pigment layer, which is heavily loaded with dark, 

 yniform, pigment granules, especially crowded together on the side of the layer 



L.sup. 



N.op. 



Vit. 



Schl. 



tu.v. 



Ret. 



L.inf. 



FIG. 223. PIG, 24.0 MM. TRANSVERSE SERIES 62, SECTION 428. 



an.ch, Anterior chamber of eye. corn, Corneal mesoderm. EC, Ectoderm (epidermis). Ep, Inner epithelium 

 of cornea. Ir, Mesodermal anlage of iris. L', Outer layer of lens. L", Inner layer of lens. L.inf, Inferior 

 eyelid. L.sup, Superior eyelid. ^.3, Oculo-motor nerve. N.op, Optic nerve. Pig, Pigment layer. Ret, 

 Retina. Schl, Canal of Schlemm. tu.v, Tunica vasculosa lentis. Vit, Vitreous humor. X 50 diams. 



nearest the retina. In later stages the pigment layer grows forward on the inner 

 side of the iris, making a fold, so that the iris is covered on the inside by a 

 double layer of pigmented epithelium, the uvea. The retina resembles closely in 

 structure the brain-wall in an early stage, for it has on its outer surface a thin 



