320 PHYSIOLOGICAL DEVELOPMENT. 



lens-shape. But the developed eye shows us a terminatk 

 of the nerve greatly expanded and divided to receive the 

 external stimulus. It shows us an introverted portion of the 

 integument containing the apparatus by which the external 

 stimulus is conveyed to the recipient nerve. The structure 

 developed in this sac not only conveys the stimulus, but also, 

 like its homologue, concentrates it; and in the one case as 

 in the other, the structure which does this is an epidermic 

 growth from the bottom of the sac. Even with the ear it is 

 the same. Again we have an introverted portion of the inte- 

 gument, on the walls of which the nerve is distributed in 

 the primitive ear. The otolithes contained in the sac thus 

 formed, are bodies which are set in motion by the vibrations 

 of the surrounding water, and convey these vibrations in 

 an exaggerated form to the nerves. And though it is not 

 alleged that these otolithes are developed from the epidermic 

 lining of the chamber, yet as, if not so developed, they are 

 concretions from the contents of an epidermic sac, they must 

 still be regarded as epidermic products. 



Whether these differentiations are due wholly to indirect 

 equilibration, or whether direct equilibration has had a share 

 in working them, are questions that must be left open. 

 Possibly a short hair so placed on a mammal's face as to be 

 very often touched, may, by conveying excitations to the 

 nerves and vessels at its root, cause extra growth of the 

 bulb and its appendages, and so the development of a vibrissa 

 may be furthered. Possibly, too, the light itself, to which 

 the tissues of some inferior animals are everywhere sensitive, 

 may aid in setting up certain of the modifications by which 

 the nervous parts of visual organs are formed : producing, as 

 it must, the most powerful effects at those points on the sur- 

 face which the movements of the animal expose to the greatest 

 and most frequent contrasts of light and shade; and propa- 

 gating from those points currents of molecular change 

 through the organism. But it seems clear that the complexities 



