DEVELOPMENT OF THE SENSE ORGANS 



381 



The mesenchyma accompanying the vessels to the proximal surface 

 of the lens, and that on its distal surface, give rise to the vascular capsule 

 of the lens (Fig. 377). On the distal surface of the lens this is supplied by 

 branches of the anterior ciliary arteries and is known as the pupillary mem- 

 brane; the vessels disappear and the membrane degenerates just before 

 birth. The artery of the lens also degenerates, its wall persisting as the 

 transparent hyaloid canal. Fibrillas extending in the vitreous humor 

 from the pars ciliata of the retinal layer to the capsule of the lens persist as 

 the zonula ciliaris, or suspensory ligament of the lens. 



Differentiation of the Optic Cup. We have seen that of the two layers 

 of the optic cup, the outer becomes the pigment layer of the retina. Pig- 

 ment granules appear in it's cells in embryos of 7 mm. and the pigmentation 

 of this layer is marked in 12 mm. embryos (Fig. 377). 



Cant 

 Rod cell 



Rod cell 

 Fiber of Miiller, 



Amacrine cell 



Ganglion cell 



Optic fibers 



External limiting 

 membrane 



Layer of rod and cone 

 cells 



i Canglionic layer 



Fibrous layer 



Internal limiting 

 membrane 





FIG. 380. Section of the nervous layer of the retina from a 65 mm. human fetus. At the left 

 is shown diagrammatically the cellular elements of the retina according to Cajal. X 440. 



The inner, thicker layer of the optic cup, the retinal layer proper, is 

 subdivided into a distal zone, the pars cceca, which is non-nervous, and 

 into the pars optica, or the true nervous portion. The line of demarcation 

 between the pars optica and the pars caeca is a serrated circle, the ora 

 serrata. By the development of the ciliary bodies the blind portion of the 

 retinal layer, the pars caeca, is differentiated into a pars ciliaris and pars 

 iridica retinas. The former, with a corresponding zone of the pigment 

 layer, covers the ciliary bodies. The pars iridica forms the proximal layer 

 of the iris and blends intimately with the pigment layer in this region, its 

 cells also becoming heavily pigmented (Fig. 379). 



The pars optica, or nervous portion of the retina, begins to differen- 

 tiate proximally and the differentiation extends distally. An outer cellular 

 layer and an inner fibrous layer may be distinguished in 1 2 mm. embryos 



