THE ADDITION OF DETAILS 



89 



animals known as Cephalopods and have hardly any 

 connection with vertebrates. And we find, if we study 

 the embryology of octopuses, that their eyes, although 

 very similar in plan to vertebrate eyes, are developed 

 in a totally different way (Fig. 23). The whole retina 

 is made from a patch of skin which sinks inwards and 



IRIS 



SKIN y^^ "'^ -LENS 



RETINA 



Fig. 23. — Development of the eye in the octopus, (a) Is a section 



of the hollow ball formed by a fold of the skin, {b) Shows the 



retina developed from this fold, and the iris from two more folds, 



one on each side of the eye 



becomes cut off from the surface, so that it forms a 

 hollow sunken sphere. Then the lens differentiates 

 from the thin roof of this sphere, and two circular 

 folds of skin grow up all round, the inner fold 

 becoming the iris, while the other may join up 

 across the top and become transparent and so change 

 into a cornea, like the vertebrate cornea, which is a 

 sort of protective eye-glass covering the pupil. The 

 octopus-brain takes no part in the whole process, 

 though it does, of course, eventually become con- 

 nected to the eye by a nerve. 



