138 THE FROG. 



The optic cup. Partly in consequence of the ingrowth of the 

 lens vesicle, but mainly through active growth of the walls of 

 the optic vesicle itself, this latter becomes pitted on its outer 

 surface, and so converted into a cup (Figs. 66, 67). This optic 

 cup, as it is termed, has double walls : the inner wall (Figs. 66 

 and 67, oc) is very thick, and consists of cells arranged three or 

 four deep ; the outer wall (Fig. 67, od) is thin, and consists of 

 a single layer of flattened cells, in which pigment is early deve- 

 loped. (Gf. Fig. 76, oc.) 



In the later stages of tadpole life the optic cup slowly 

 enlarges ; it remains in contact with the lens at its edge or lip, 

 but elsewhere is separated from this by a space, which becomes 

 the posterior chamber of the eye, and in which the vitreous body 

 is formed. 



The inner, or thicker, wall of the optic cup gives rise to the 

 retina ; the molecular and nuclear layers, and the layers of nerve 

 cells and nerve fibres, being formed by modification of the cells 

 of the wall itself ; while the rods and cones of the bacillary layer 

 arise as outgrowths from its outer surface, which grow towards, 

 and become imbedded in, processes developed from the pig- 

 mented cells of the outer wall (Fig. 67, od) of the optic cup. 



If the mode of development of the brain be called to mind, it 

 will be seen that the layer of epithelial cells which lines the 

 cavity, or ventricle, of the fore-brain (Fig. 67, bf) is morpho- 

 logically equivalent to the outer or epidermic layer of the 

 surface epiblast, and was originally directly continuous with 

 this, before closure of the neural groove was effected. As the 

 optic vesicle is an outgrowth from the fore-brain, the cells lining 

 its cavity, i.e. the cells lining the space between the inner, oc, 

 and outer, od, walls of the optic cup, will be of the same nature 

 as those lining the cavity of the fore-brain itself. 



The optic nerve. The fibres of the optic nerve are developed 

 on the inner surface of the inner layer of the optic cup, i.e. 

 the surface next to the vitreous body, and grow inwards along 

 the optic stalk to the brain. It follows from what has been said 

 in the previous paragraph that this inner surface of the optic 

 cup is morphologically equivalent to the deeper or nervous layer 

 of the epidermis, from which we have seen that all the other 

 nerves are developed, directly or indirectly. 



Up to the time of hatching there is no trace of the optic 



