38 



DEGENERATION : 



shape, whilst the cloacal chamber forms. The brain 

 remains quite small and undeveloped, and the remark- 

 able myelonic eye (the eye in the brain) disappears. 

 The number of gill-slits increases as the animal grows 

 in size and its outer skin becomes tough and leather- 

 like. 



Before saying anything further on the subject of 

 degeneration, it seems desirable once more to direct 



FIG. 21. FIG. 22. 



FIG. 21. Section through the eye ("surface eye") of a Water-beetle's larva. 

 All the cells are seen to be in a row continuous with h, the cells of the outer- 

 most skin or ectoderm, p, pigmented cells ; r, retinal cells connected at o 

 with the optic nerve; g, transparent cells (forming a kind of "vitrequs 

 body") ; I, cuticular lens. (From Gegenbaur's Elements of Comparative 

 Anatomy, after Grenacher. ) 



FIG. 22. Section through the eye ("surface- eye") of a Marine Worm (Neo- 

 phanta). 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. 



attention to the myelonic or cerebral eye which the 

 Ascidian tadpole possesses in common with all Ver- 

 tebrates. All other animals which have eyes develop 

 the retina or sensitive part of the eye from their outer 

 skin (see Figs. 21 and 22, and explanation). It is 



