DEVELOPMENT OF THE SENSE ORGANS 



381 



derivative of both retinal and lens cells, it forming a non-cellular network of cytoplasmic 

 processes which are continuous with the cells of the lens and retina. With the ingrowth of 

 the central artery of the retina, from which the artery of the lens passes to the proximal sur- 

 face of the lens and branches on it, a certain amount of mesenchymal tissue invades the optic 

 cup, and this tissue probably contributes to the development of the vitreous body (Fig. 377). 

 The vitreous body may therefore be regarded as a derivative both of the ectoderm and 

 of the mesoderm. 



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 membrane; the vessels 

 disappear and the membrane degenerates just before birth. The artery of the 



Cone ai! 



Rod cdi 



Rod cell 

 Fiber of MiUler 



Amacrine cell 



Ganglion cell 



Optic fibers 



„--^j^—Extcriinl limiting 

 memhyiine 



La\er of rod and cone 



ails ■ 



■M©i^5?Tf — Ganglionic layer 



Fibrous layer 



All ;i!i ^^IJ ''. ,^'ifr v'\ Internal 



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. 



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 cap- 

 sule 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. Pigment granules 

 appear in its cells in embryos of 7 mm. and the pigmentation of this layer is 

 marked in 12 mm. embryos (Fig. 377). 



The inner, thicker layer of the optic cup, the retinal layer, is subdivided into 

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

 nervous retina proper. The line of demarcation between the pars optica and the 

 pars CEeca is a serrated circle, the ora serrata. The bUnd portion of the retinal 

 layer, the pars caeca, with the development of the cihary bodies is differentiated 



