GENERATION AND DEVELOPMENT. 293 



same fissure sometimes occurs in man under the name coloboma iridis. 

 The cavity of the primary optic vesicle becomes completely obliterated, 

 and the rods and cones come into apposition with the pigment layer of 

 the retina. 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 



FIG. 451. FIG. 452. 



FIG. 451. Diagrammatic sketch of a vertical longitudinal section through the eyeball of a human 

 f ostus of four weeks. The section is a little to the side, so as to avoid passing through the ocular 

 cleft; c, the cuticle where it becomes later the corneal epithelium ; Z, the lens; op, optic nerve formed 

 by the pedicle of the primary optic vesicle; vp, primary medullary cavity or optic vesicle; p, the 

 pigment layer of the retina; r, the inner wall forming the retina proper; t>s, secondary optic vesicle 

 containing the rudiment of the vitreous humor, x 100. (Kolliker.) 



FIG. 452. Transverse vertical section of the eyeball of a human embryo of four weeks. The an- 

 terior half of the section is represented; pr, the remains of the cavity of the primary optic vesicle; 

 p, the inner part of the outer layer forming the retinal pigment; r, the thickened inner part giving 

 rise to the columnar and other structures of the retina; v, the commencing vitreous humor within 

 the secondary optic vesicle ; i/, the ocular cleft through which the loop of the central blood-vessel, 

 a, projects from below; Z, the lens with a central cavity, x 100. (Kolliker.) 



the corneal tissue proper is derived from the mesoblast which intervenes 

 between the epiblast and the primitive lens which was originally continu- 

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

 general mesoblast in which it is imbedded. 



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

 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 mem- 

 brane, the membrana pupittaris, which forms the front portion of a. 

 highly vascular membrane that, in the foetus, surrounds the lens, and is 

 named the membrana capsulo-pupillaris (Fig. 453). It is supplied with 

 blood by a branch of the arteria centralis retincv, which, passing forward 

 to the back of the lens, there subdivides. The membrana capsulo-pupil- 

 laris withers and disappears in the human subject a short time before 

 birth. 



The eyelids of the human subject and mammiferous animals, like those 

 of birds, are first developed in the form of a ring. They then extend 

 over the globe of the eye until they meet and become firmly agglutinated 



