208 DEVELOPMENT OF THE TADPOLE. 



walls of the sac being formed of a series of almost rectangular 

 cells, placed side by side, being transformed epiblastic cells. 



In PI. XIX. and Figs. 3, 6, 7, 8, and 9, PI. XX., respectively, will 

 be found diagrams and sketches of the eye at various periods. 

 Figs. 6 and 7, PI. XX.^ are diagrammatic representations of the 

 evolution of the eye, as usually given in text-books on physiology, 

 the remaining figures being drawn from various sections of my 

 own to illustrate the appearances presented by this organ at 

 different dates. 



Lankester, who first worked out the development of this organ, 

 states that it appears as an oval pit of the epiblast, the edge of 

 which is formed by a projecting rim. The edges of this pit gra. 

 dually approach each other, whilst the epiblast lining of the floor 

 thickens. The cavity thus formed is eventually converted into the 

 inner optic chamber. A layer of mesoblast next grows in between 

 the walls of the sac and the external epiblast. The iris and lens 

 arise nearly simultaneously, the latter appearing to proceed from 

 the epiblast. In Fig. 8, PL XX., and Fig. 3, PI. XIX. , this lens 

 may be seen situated between the horns of the invaginated epi- 

 blast, but, as shown in the latter figure, now covered completely 

 by an epiblastic layer, transformed into skin. In both these 

 figures the nerve-cords proceeding from the brain to the eye can 

 be distinctly traced, and in Fig. 5, PI. XIX., we have a represen- 

 tation of the roots of these nerves as they proceed from the brain 

 itself. The interior of the eye at this date is filled with a transpa- 

 rent substance radiating from the exterior to the interior, where 

 most, if not all, of these almost transparent fibres terminate in a 

 small knob quite as transparent as themselves. 



In his article -on the " Development of the Cephalopoda," 

 Lankester gives an account of the first formation of the lens of 

 the eye. He says : — " It is formed entirely within the primitive 

 optic chamber, and at first depends as a short, cylindrical rod 

 from the middle point of the anterior wall of that chamber — that 

 is to say, from the point at which the chamber is finally closed up. 

 It grows subsequently by the deposition of concentric layers of a 

 horny material round this cone. No cells appear to be imme- 

 diately concerned in effecting the deposition, and it must be 

 looked upon as an organic concretion, formed from the liquid 



