266 Br. R. M. Gunn on the 



A little later a cavity first becomes visible on the optic 

 outgrowth and on the central nervous mass, and consequently 

 at this stage the developing eye may, for the first time, be 

 truly described as an optic vesicle. The outer wall of this 

 vesicle is about twice the thickness of the inner, the former 

 being about four cells and the latter about two cells deep. 

 The cells in both walls are similar in appearance, being of 

 an oval form and uniform size, and they are arranged 

 radially, t. e. with their long axes perpendicular to the slit- 

 like cavity. The only exception to this arrangement is ante- 

 riorly and posteriorly, where the two walls become continu- 

 ous with one another ; in these situations the radial disposition 

 seems not to exist. The nuclei of the inner wall of the 

 vesicle are rather more deeply stained than those of the outer. 

 The cells of the developing brain are very like those of the 

 optic vesicle, and are similarly arranged in relation to its 

 central cavity. At this stage is also observed a slight thick- 

 ening of the deep layer of the cuticular epiblast corresponding 

 to the position of the future lens ; but this is not so far 

 advanced as to indent the outer wall of the vesicle. A pro- 

 longation of cells from the same deep layer of cuticular epi- 

 blast is now observed lining the sides and base of the optic 

 vesicle, so as to separate it from the brain. 



The cavity of the optic vesicle seems soon to disappear, for 

 at the next stage examined, while there are few important 

 changes, there is only a faint line of separation between the 

 outer two thirds and the inner one third of the cells. Internal 

 to the vesicle there is a double layer of elongated cells, their 

 long axes being parallel to the wall of the vesicle. One of 

 these layers follows the curve of the vesicle, while the other 

 (inner) layer is closely applied to the central nervous mass. 

 Where the rows diverge anteriorly large cells from below the 

 cuticular epiblast dip into the angle so formed. The lens 

 develops rapidly and is already well advanced, indenting the 

 outer wall of the optic outgrowth, and so forming a " secon- 

 dary " optic vesicle. At this stage no pigment-granules are 

 discernible. 



Differentiation now soon occurs in the optic outgrowth, so 

 as to indicate roughly the position of the future layers. We 

 may therefore in future speak of it as " retina," and describe 

 the position of the layers in relation to the secondary optic 

 vesicle or optic cup instead of to the outer wall of the embryo 

 as we have hitherto done. A series of elongated deeply 

 staining cells, aa-ranged closely side by side and with their 

 long axes radial, form a single row most externally in the 



