

DEVELOPMENT OF THE RETINA. 295 



a very distinct lamination becomes apparent in what was ori- 

 ginally the homogeneous mass of the (anterior) retina. This 

 lamination consists in the differentiation of an internal fibrous 

 layer, of the two granulated layers, and of distinct differences in 

 the size of the cells in the several layers of the granules and 

 ganglion cells ; at the same time the rudiments of the rods and 

 cones project posteriorly beyond the membrana limitans ex- 

 terna in the form of homogeneous hemispherical elevations of 

 very small diameter. As these increase in length and thick- 

 ness the internal segment is first formed, and then subsequently 

 the external segment. These consequently grow into the pig- 

 mented epithelial cells of the posterior lamina of the retina, 

 which on their side form the pigment sheaths. On the eighteenth 

 day of incubation, coloured, but at first very small, red and 

 subsequently yellow oil globules appear in the cones, so that 

 the retina of the Chick just escaped from the egg is already 

 provided with completely developed percipient elements, which 

 do indeed increase in length and thickness, but not in number. 

 It is moreover worthy of notice that in Chickens the rods and 

 cones appear from the first as distinct structures, and that the 

 cones, which are originally smaller than the rods, immediately 

 after hatching become considerably thicker, and, with their 

 coloured globules, occupy a much larger space than at an earlier 

 period. 



The relation of the developing rods and cones to the external 

 granules has been ascertained by Babuchin from his researches 

 on the retina of the Tadpole.* The large size of the element- 

 ary parts permits it to be seen that the rods and cones owe 

 their origin to an outgrowth of the substance of the outer gra- 

 nules, and although when fully developed the rods and cones 

 appear very distinct, the difference, according to Babuchin, in 

 the early stages of development is but slight. 



The results of the investigations made by Schenk on Fishes 

 agree well with these observations on the development of the 

 rods and cones from the anterior lamina of the primary eye 

 vesicle. This process, so far as it is connected with the pro- 

 duction of cells on one side of a substance differentiated from 



* Loc. cit.j p. 77. 



