DEVELOPMENT OF THE SENSE ORGANS. 



893 



thickening of the epiblast (I, L). It is hollow, and as it grows inwards ultimately becomes 

 spherical and separated from the epiblast to form the crystalline lens, so that the lens is epi- 

 blastic in its origin, while the capsule of the lens is a cuticular structure formed from the epiblast. 

 That part of the epiblast which covers the vesicle in front of the lens ultimately becomes the 

 stratified epithelium of the cornea. The layer of pigment of the invaginated optic vesicle is 

 applied to the ciliary body, and the posterior surface of the iris, when the latter is formed. 

 The cornea is formed at the 6th week. The substance of the choroid, sclerotic, and cornea is 

 formed around the position of the eye from the mesoblast (m). The capsule of the lens is 

 at first completely surrounded by a vascular membrane the membrana capsulo-pupillaris. 

 Afterwards, the lens passes more posteriorly into the eye the anterior part of the capsulo- 

 pupillary membrane, however, remains in the anterior part of the eye, while towards it grows 



Fig. 691. 

 Development of the eye. I., Inflexion of the sac of the lens (L) into the primary optic vesicle 

 (P) e, epidermis ; m, mesoblast. II., The inflexion seen from below n, optic nerve ; e, 

 the outer, i, the inner layer of the inflected vesicle ; L, lens. III., Longitudinal section 

 of II. IV., Further development e, corneal epithelium ; c, cornea ; m, membrana 

 capsulo-pupillaris ; L, lens ; a, central artery of the retina; s, sclerotic ; ch, choroid ; p, 

 pigment layer of the retina ; r, retina. V., Persistent remains of the pupillary membrane. 



the margin of the iris (7th week), so that the pupil is closed by this part of the vascular capsule, 

 membrana pupillaris. The blood-vessels of the iris are continuous with those of the pupillary 

 membrane ; those of the posterior capsule of the lens give oft* the hyaloid artery, a continuation 

 of the central artery of the retina ; its veins pass into those of the iris and choroid. The 

 vitreous humour at the 4th week is represented by a cellular mass between the lens and the 



Fig. 692. 

 Early stages in the development of the vertebrate ear. A-D, Early stages in the chick (Reissiier). 

 E, Transverse section through the auditory pit of a 50 hours' chick (Marshall). F, Trans- 

 verse section through the hind-brain of a foetal sheep, acv, anterior cardinal (jugular) 

 vein ; am, amnion ; ao, aortic arch ; ce, cochlea ; rv, recessus (aqueductus) vestibuli ; v, 

 vestibulum ; vc, vertical semicircular canal ; viii, auditory nerve. 



retina. The pupillary membrane disappears at the 7th month. It may remain throughout 

 life (V). 



Organ of Smell. On the under surface and lateral limit of the fore-brain, the epiblast forms 

 a groove or pit with thickened epithelium, which forms a depression towards the brain, but 



