THE ORGANS OF SPECIAL SENSE. 



571 



(Figs. 501 , 505) . Further development consists in a gradual differentiation, within 

 the broad layer, of the various retinal elements and consequent demarcation of the 

 layers which constitute the adult retina. The next layer to differentiate is the 

 innermost layer of the retina, or layer of nerve fibers. This appears during the 

 sixth or seventh week as a thin, clear, faintly striated zone containing a few 

 scattered nuclei. What remains of the original inner layer of the cup has now 

 become a comparatively thick layer with numerous chromatic and actively 

 dividing nuclei. It may be conveniently designated the primitive nuclear layer. 



Surface epithelium 



of eyelid 



Eyelid (upper) 



Corneal epithelium 



Conjunct! val 



epithelium 



Substantia 



propria corneae 



Lens 



Anterior epithe- 

 lium of lens 



Conjunctival sac 



Chorioid 

 Pigmented layer 

 of retina 

 Split between 

 retinal layers 

 Retina, except 

 pigmented layer 

 Vitreous 



Tunica vasculosa 



lentis 



Nerve fiber layer 



of retina 



Hyaloid artery 



Central artery 

 of retina 



Optic nerve 



FIG. 505. Horizontal section through eye of human embryo of 13-14 weeks. Modified from Lange. 



The similarity in development between the retina and wall of the neural tube 

 is to be noted. Thus the layer of nerve fibers appears to correspond quite 

 closely to the marginal layer of the central nervous system, while the primitive 

 nuclear layer is probably homologous with the mantle layer (pp. 479, 485). 

 There is a similar correspondence between the retina and the central nervous 

 system in regard to their early cellular development, the retinal cells early 

 showing a differentiation into neuroblasts and spongiobiasts (pp. 479, 485). 



About the end of the eighth week the inner part of the primitive nuclear 

 layer differentiates into the layer of ganglion cells (Fig. 506, It). These 

 are large cells and with their processes constitute the third or proximal optic 

 neurone. They can be first distinguished in the fundus of the cup and gradu- 

 ally extend to the ora serrata. They are the first of the cellular elements of the 

 adult retina which can be definitely recognized as such. From each cell, two 

 kinds of processes develop, dendrites, which ramify in this and in the more 

 external layers of the retina, and an axone which grows toward the cavity of 

 the eye and becomes a fiber of the layer of nerve fibers, whence it continues into 



