DEGENERA TION. 



47 



Before saying anything further on the subject of 

 degeneration, it seems desirable once more to direct 

 attention to the myelonic or cerebral eye which the 

 Ascidian tadpole possesses in common with all Ver- 

 tebrates. All other animals wdiich have eyes 

 develop the retina or sensitive part of the eye from 

 their outer skin (See Figs. 21 and 22, and explana- 

 tion.) It is easy to understand that an organ which 



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 outermost skin or 

 ectoderm. /, pigmented cells; r. retinal cells connected at o with the optic 

 nerve; g. transparent cells (forming a kind of " vitreous body ") ; /, cuticular 

 lens. (From Gegenbaur's "Elements of Comparative Anatomy,' ' after Grenacher.) 



is to be affected by the light should form on the 

 surface of the body where the light falls. It has 

 long been known as a very puzzling and un- 

 accountable peculiarity of Vertebrates, that the 

 retina or sensitive part of the eye grows out in 

 the embryo as a bud or vesicle of the brain, and 

 thus forms deeply below the surface and aivay from 



