262 STUDY OF PIG EMBRYOS. 



of the cornea and the anterior surface of the lens is a clear space, an. ch^ which we 

 can identify as the anterior chamber of the eye, which in the adult is filled only 

 with the aqueous humor. On the corneal side the anterior chamber is bounded 

 by a distinct layer of cells, Ep, the internal epithelium of the cornea. This 

 layer is, however, formed from the mesenchyma, the cells of which develop 

 into the internal epithelioid covering of the cornea. At the upper and lower 

 edge of the cornea there is a separate forward growth, Ir, of the connective 

 tissue between the cornea and the lens. It is the anlage of the connective- 

 tissue layer of the iris. In later stages it will grow still further over the 

 lens from all sides, leaving a central opening, the pupil, and it will acquire 

 a special pigmented layer on its side nearest the lens. At the base of the 

 iris anlage is a small blood-vessel, Schl, which is commonly designated in 

 the adult as the canal of Schlemm. The retina has increased in thick- 

 ness and is closely covered by a pigment layer, Pig. The separation which 

 appears on the inner side of the eyeball between the retina and pigment layer 

 in figure 150 is probably artificial, the result of shrinkage during the preserva- 

 tion 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, uniform, pigment granules, especially crowded together on the side 

 of the layer nearest the retina. In later stages the pigment layer grows for- 

 ward 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 layer corresponding to the ectoglia, and within a broad, nucleated zone. 

 The mitotic figures are found only next to the surface of the retina nearest the 

 pigment layer. Since the space between the pigment layer and the retina corre- 

 sponds to the cavity of the brain, it is evident that the position of the mitotic 

 figures is homologous with their position in the medullary wall elsewhere. The 

 section of the lens clearly reveals its vesicular structure. The external wall of 

 the lens vesicle, L", is a comparatively thin epithelial layer which stains quite 

 readily and therefore stands out clearly in the section. Toward the edges of the 

 lens the outer layer slightly thickens and then passes over quite abruptly into 

 the inner layer of the vesicle, L', which is very thick and constitutes by far the 

 greater part of the bulk of the organ and gives to the lens its characteristic shape. 

 The outer and inner walls of the lens are in close contact so that there is no actual 

 cavity present. The epithelial cells of the inner wall have elongated enormously, 

 so much that they might perhaps already be termed " fibers." Each cell is sup- 

 posed to extend through the entire thickness of the inner wall. The nuclei are 

 placed somewhat irregularly in the middle portion of the long cells so that they 



