766 



DEVELOPMENT OF THE ORGANS OF SENSE. 



retinal and pigmental layers rapidly contracts, and finally the rods and 

 cones are closely united ^Yith the layer of pigment cells. 



The optic nerve, as already described, is at first connected by its 

 origin with the vesicle of the third ventricle or thalamencephalon, and 

 for a time it retains its original hollow form. But as the cerebral 

 hemispheres are developed forwards, the eye and the optic nerve are 

 thrown backwards and downwards, and a new connection is estal^lished 

 between the optic nerve (or tract) and the vesicle of the midbrain 

 (mesencephalon) : the rudiment of the optic commissure is at the 

 same time formed by the median approximation of the stalks and the 

 growth of one over the other. Each stalk then becomes more and more 

 solid by the development of nerve fibres along with the intruded 

 connective tissue which forms the sheath substance of the nerve. 



Lens. — The development of fibres from the hinder wall of the 

 primitive lens-follicle continuing to take place, the cavity of the 

 follicle is first greatly narrowed and then completely filled up by the 

 lengthening fibres, and the lens takes more and more of its fall 

 spherical shape. The new fibres continue to be formed towards the 

 margin of the lens ; each fibre retaining its nucleus, so as to produce 

 the "nuclear zone which runs through the whole lens. This zone 

 is at first situated far back in the lens while the fibres are still 

 short, but as they elongate, its place is advanced, so that it comes to 

 be situated considerably in front of the equatorial plane of the lens. 

 It is most distinct towards the margin where the fibres are newly 

 formed. The anterior wall of the lens-follicle remains as a simple 

 cellular layer. The greater number of the fibres now follow the general 

 curve of the surface of the lens, presenting therefore their concavity 

 towards its centre, but the curvature gradually diminishing in those 

 nearest the middle, where they are straight or nearly so. Only the 

 external short and recently formed fibres present a concavity towards 

 tlie exterior. The intersecting stars of the anterior and posterior poles 

 of the lens now make their appearance by the collection of cells in the 

 peculiarly shaped triradiate space in these two situations, and the 

 ends of the fibres are now traceable to the edges of these spaces, so that 

 the fibres gradually take the arrangement round the poles of the lens 

 which belongs to the adult. 



Fig. 567.— Transverse Vertical Section of the 

 Eyeball op a Human Embryo of four weeks 

 (from Kolliker). '"" 



The anterior half of the section is represented. 

 pr, the remains of the cavity of tlie primary optic 

 vesicle ; ii, the inflected part of the outer layer, form- 

 ing the retinal pigment ; r, the thickened inner 

 part giving rise to the columnar and other struc- 

 tures of the retina ; r, the commencing vitreous 

 humour within the secondary optic vesicle ; v', the 

 ocular cleft through which the loop of the central 

 blood-vessel, a, projects from below ; 1, the lens 

 with a central cavity. 



The capsule of the lens, according to Lieberkiihn's and Julius Arnold's 

 most recent observations, owes its origin to the thin innermost pellicle 

 of mesoblast which is introduced atan early period of development 

 between the lens and the secondary ocular vesicle. 



