8H HANDBOOK OF PHYSIOLOGY. 



The inner segments of the. rods become the first formed, then the outer. 

 The cavity of its pedicle disappears and the solid optic nerve is formed. 

 Meanwhile the cavity which existed in the centre of the primitive lens 

 becomes filled up by the growth of fibres from its posterior wall. The 

 epithelium of the cornea is developed from the epiblast, while the cor- 

 neal tissue proper is derived from the mesoblast which intervenes between 

 the epiblast and the primitive lens which was originally continuous 

 with it. The sclerotic coat is developed round the eyeball from the 

 general mesoblast in which it is embedded. The choroid is developed 

 from the mesoblast on the outside of the optic cup and the iris by the 

 growing forward of the anterior edge of the optic cup, both layers of 

 which becoming pigmented remain as the uvea. Externally the cho- 

 roidal mesoblast grows inward to form the main structure. The ciliary 

 processes arise from the hypertrophy of the edge of the optic cup which 

 forms folds into which the choroidal mesoblast grows, and in which 

 blood-vessels and pigment-cells develop. 



The iris is formed rather late, as a circular septum projecting in- 

 ward, from the fore part of the choroid, between the lens and the 

 cornea. In the eye of the foetus of mammalia, the pupil is closed by a 

 delicate membrane, the menibrana pupillaris, which forms the front por- 

 tion of a highly vascular membrane that, in the foetus, surrounds the 



Fig. 507. Blood-vessels of the capsulo-pupillary membrane of a new-born kitten, magnified. 

 The drawing is taken from a preparation injected by Tiersch, and shows in the central part the 

 convergence of the net-work of vessels in the pupillary membrane. (Kolliker.) 



lens, and is named the menibrana capsulo-pupillaris (fig. 507). It is 

 supplied with blood by a branch of the arteria centralis retince, which, 

 passing forward to the back of the lens, there subdivides. The mem- 

 brana capsulo-pupillaris withers and disappears in the human subject a 

 short time before birth. 



The eyelids of the human subject and mammiferous animals, like 



