STUDY OF SECTIONS OF EMBRYO OF 24 MM. 331 



retina and pigment layer. The origin of the retina and lens is illustrated by the 

 chicken embryo (Figs. 153, 154), and in a more advanced stage by the pig of 

 12 mm. (Fig. 192). There is added here figure 222, from a section of the eye 

 of a rabbit embryo of thirteen days, in order to facilitate the comparison between 

 the 12 mm. stage and the 24 mm. stage of the pig. In figure 222 the ectoderm, 

 EC, forms an arch over the eye and indicates the commencing formation of the 

 cornea, the layer of ectoderm being destined to become the external epithelium 

 of the cornea. Between the lens and the retina there has been an ingrowth of 

 tissue accompanied by blood-vessels, which forms a more or less distinct covering 

 over the surface of the lens and constitutes the so-called tunica vasculosa, tu.v. The 

 space between the retina and lens will increase during the following stages and 

 will become occupied by a very clear tissue containing a minimal number of cells. 

 This "clear tissue is the commencement of the vitreous humor. Between the lens 

 and the overlying ectoderm the mesenchyma has begun to penetrate. This mesen- 

 chyma will ultimately furnish the connective tissue of the cornea and of the iris. 

 About the eyeball as yet there is no distinct condensation of tissue such as will 

 appear later to develop the anlages of the choroid and scleral coats of the eyeball. 

 In the pig of 24 mm. (Fig. 223) we encounter a marked advance in the differ- 

 entiation of all parts of the eye. Above and below the eye the anlages of the 

 eyelids, L.sup, L.inf, have appeared. The anlage is at this stage merely a projecting 

 fold of the ectoderm filled with mesenchyma and extending a short distance over 

 the projecting cornea. -The folds will continue to grow until the eyelids meet in 

 the middle of the eye, covering it completely. The ectoderm of the two lids where 

 they meet unites. The union of the two lids occurs in all mammals, and in some 

 cases they do not separate again until after birth, in which case the animals are 

 said to be "born blind." The ectoderm, EC, of the cornea describes a wide, high 

 arch, underneath which is a broad band of embryonic connective tissue, corn, 

 which forms the main thickness of the cornea. Between the connective tissue 

 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 farther over the lens from all sides, leaving a cen- 

 tral 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 thickness and is closely covered by a pigment layer, Pig. The separa- 

 tion which appears on the inner side of the eyeball between the retina and pig- 



