38 



DEGENERATION : 



shape, wliilst tlie 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 



WWl 



Fig. 21. Fig. 22. 



Fig. 21.— Section through the eye ("surface eye") of a Water-beetleV- larva. 

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

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

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

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

 Anatomy, after Greuacher. ) 



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

 phanta). i, integument spreading over the front of the eye c ; I, cuticular 

 lens ; h, cavity occupied by vitreous body ; xj, retinal cells ; h, 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 



