DEVELOPMENT OF EYE OF SPARROW 269 



the cells forming the posterior part of the capsule. The epithe- 

 lium covers the lens, the cornea has not developed, and the 

 anterior chamber of the eye has not yet been formed. 



Since the brain arises from the ectoderm, the two layers form- 

 ing the optic cup are of ectodermal origin. The uvea of the iris 

 and the pigment of the ciliary bodies, since they are derived 

 from the retina, are likewise ectodermal. The lens is developed 

 directly from the ectoderm. The thin layer of ectoderm covering 

 the embryonic lens later becomes the conjunctival epithelium. 

 All other structures of the eye — the chorioid, iris, sclera, muscles 

 (the muscles of the iris possibly excepted), blood-vessels, etc. — 

 are derived from the mesoderm. 



THE CORNEA, IRIS, AND AQUEOUS CHAMBER 



For some time after the formation of the lens vesicle the front 

 of the eye is covered only by the ectoderm, which later becomes 

 the conjunctival epithelium of the cornea. Little change is 

 seen in the development of the cornea, iris, and aqueous chamber 

 of the sparrow up to the second day of incubation. 



Figure 31, which represents the development at the age of two 

 days, shows that the conjunctival epithehum (c) is the only 

 portion of the cornea present and that it is applied closely to 

 the lens. Neither the iris nor the aqueous chamber shows any 

 development. It is true that there is an open space between 

 the lens (L), conjunctival epithelium (C), mesoderm (m), and 

 the pigment portion of the retina (P), but this can scarcely be 

 considered the beginning of the aqueous chamber. A slight 

 projection from the mesoderm (Md) is found. This is the 

 beginning of the posterior layer of the cornea, or the membrane 

 of Descemet. 



Hertwig ('90) says that in the chick as early as the fourth day 

 a thin structureless sheet of mesenchyme extends between the 

 lens and the epidermis, into which numerous mesenchymatous 

 cells later migrate from the margin and become the corneal 

 corpuscles. He further states that these corpuscles later form 

 the corneal fibers, and the structureless sheet forms the cement- 

 ing substance between the fibers, the membrana elastica anterior, 

 and the membrane of Descemet. 



