DEVELOPMENT OF THE OPTIC NERVE OE VERTEBRATES. 97 



bundle arise as outgrowths from neuroblasts (fig. 13, N., 

 Plate XII) near the rim of the optic cup, for along this rim 

 a constant proliferation of cells takes place as long as the eye 

 increases in size. 



Fig. 13 shows the main features in the structure of the 

 retina of a tadpole of 13 mm,, and is as far as possible a 

 camera drawing. 



Towards the centre of the cup the development of the 

 retina is furthest advanced. 



The two walls of the optic vesicle are closely approximated, 

 and the originally inner wall is reduced to a single layer of 

 pigmented cells (P.). 



These pigmented cells are directly continuous with the 

 walls of the optic stalk, and thereby with the epithelial lining 

 of the neural tube. These cells represent the epidermic layer 

 of the epiblast, so that from this part of the optic cup it will 

 be seen that the nervous layer is entirely absent. 



In the originally outer wall of the optic vesicle there are 

 many cells derived from the nervous layer, and the epidermic 

 cells are greatly elongated and to a certain extent branched, 

 and form the supporting elements only of the retina. 



The relation of these two parts is shown diagrammatically 

 in fig. 12, which represents a rather earlier stage (tadpole, 

 9 ram.). 



The further discussion as to the fate of the nervous and 

 epidermic layers of epiblast I defer to a paper on the develop- 

 ment of the central nervous system of the frog, on which I 

 am at present engaged. 



I think, however, that there is no doubt that the spon- 

 gioblastic elements are all derived from the epidermic layer, 

 and the neuroblastic from the nervous layer of the primitive 

 epiblast. 



To return to the section. Next to the pigment layer are 

 seen the developing rods and cones. The former are more 

 developed than the latter, and already their processes, which 

 project into the cavity of the optic vesicle, show the division 

 into inner and outer limbs. 



