Eyes of Mollnscs und Arthropods. 601 



of whieli soon lose tlieir color, aucl, at the outer Surfaee of tlie cells, as- 

 sume a reg'ular arrang-cment in layers. Witli tlieir loss of color and furtlier 

 developuieut. tlie grauulcs become more refractive and flattcued, uutil 

 tlicy fiually unite to form tlie membraues of Square plates, so cbarac- 

 teristie of the arg-entea in the adult. By the overlapping of the cells (tig. 5), 

 the arg-entea is soon converted into a distinct double layer, of which the 

 outcr cells become raost quiekly transformed into the reflecting mem- 

 brane . while the inner retain for a long time their nucleated . proto- 

 plasmic structure. 



The retinal layer (fig. 5, r) increases in thickness, and its large. 

 deeply stained, oval nuclei, containing numerous dark g-rauules, become 

 two or three layers deep ; at the same time a slight indentation on the 

 outer surface iudicates the beginning of the saucer-shaped depression 

 of the retina which, in the adult, is lined with the outer ganglionic cells. 

 An extremely delicate structureless membrane forms the boundary of 

 this depression and furnishes the first trace of the septum (fig. 5, s). 

 Just above the latter, the nuclei of four eounective tissue cells are col- 

 lected. being the first step toward the formation of the lens. The fiirther 

 developuieut of the retina is accomplished by an increase in the depth 

 of the central depression , below which the outer ganglionic layer 

 becomes established by the arrangement of the nuclei in a single layer, 

 and the appearance of the celi walls. The nuclei , on the thickened 

 periphery of the retinal layer, are stili characterised by being more 

 deeply stained, and arrauged three or four deep. As these changes 

 become more and more marked, the retina divides into three zones. 

 consisting of the fibrous (fig. 17?/) , the ganglionic (r), and the retinophoric 

 layer 'n.rf.). The fibrous layeris first seen beneath the septum, as a clear 

 area, which gradually increases in width, until it has reached the con- 

 dition represented in fig. 17. The ganglionic cells, which in the earlier 

 stages we bave seen separated from the hypodermic core, cau no louger 

 be distinguished; but, from the tissue of the Ophthalmie fold arise nume- 

 rous. varicose, nerve fibres which, reaching the eye, penetrate the wall 

 of the optic vesicle, where they divide into numerous branches. These 

 nerve fibres are the remnants of the outer ends of ganglionic cells, 

 which — after the withdrawal of their nuclei and celi bodies from the eye 

 into the underlying tissue — stili remain unitedwith the retina by a long 

 nerve fibre, the end of which iudicates, approximately, the originai posi- 

 tion of the ganglionic cells (figs. 4 and 5 n.f). With the developmeut 

 of the retina these fibres increase in number and assume a more parallel 

 arrangement. Whereas at first the fibres seemed to terminate indefin- 



