48 



DEGENERA TION. 



the light (see Fig. 23, and explanation). The Ascl- 

 dian tadpole helps us to understand this, for it is 



Fig 22.— Section through the eye (" surface-eye ") of a Marine Worm (Neophanta). 

 i, integument spreading over the front of the eye c ; I, cuticular lens ; h, cavity 

 occupied by vitreous body; p, retinal cells; b, pigment; o, optic nerve: o'. 

 expansion of optiC nerve. 



3 



Fig. 23. — A. Vertical section through the head of a verj' young fish, showing in the 

 centre the cavity of the brain c. Ot\ each side is a hollow outgrowth {a) which 

 will form the retina of the fish's eye ("cerebral eye"); b, will become the 

 optic nerve connecting the brain and the retina ; d, integument. — B. Later con- 

 dition of the hollow outgrowth {a) oi A. Its outer wall r is pressed against its 

 deeper wall/ by an ingrowth (I) from the outer skin (ectoderm) e; r, gives rise to 

 the retinal cells, whilst only /, the cellular lens, is derived from the surface of the 

 skin. 



