A CHAPTER IN DARWINISM 



39 



easy to understand that an organ which 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 unaccountable 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 

 away from the light (see Fig. 23, and explanation). 

 The Ascidian tadpole helps us to understand this, for 



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

 the centre the cavity of the brain c. On 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 condition of the hollow outgrowth (a) of A. Its outer wall r is pressed 

 against its deeper wall j) by an ingrowth (Z) from the outer skin (ectoderm) 

 e ; r gives rise to the retinal cells, Avhilst only I, the cellular lens, is derived 

 from the surface of the skin. 



it is perfectly transparent and has its eye actually 

 inside its brain. The light passes through the trans- 

 parent tissues and acts on the pigmented eye, lying- 

 deep in the brain. We are thus led to the suggestion 

 — and I believe this inference to be now for the first 

 time put into so many words — that the original Ver- 

 tebrate must have been a transparent animal, and had 

 an eye or pair of eyes inside its brain, like that of the 



