346 PHYSIOLOGK AL DEVELOPMENT , 



constiiutional tendency to form bone had been established by 

 actions of the kind described ; for it is a familiar fact that 

 differentiated types of tissue, having once become elements 

 of an organism, are apt occasionally to arise in unusual 

 places, and there to repeat all their peculiar histological cha- 

 racters. And this may possibly be the reason wh}^ the bones 

 of the skull, though not exposed to forces such as those which 

 produce, in other bones, dense outer la3'ers including less 

 dense interiors, nevertheless repeat this general trait of bony 

 structure. While, however, it is beyond doubt that some 

 bones are not due to the direct influence of mechanical stress, 

 we may, I think, conclude that mechanical stress initiates 

 bone-formation. 



§ 302. What is the origin of nerve ? In what way do its 

 properties stand related to the properties of that protoplasm 

 whence the tissues in general arise ? and in what way is it 

 differentiated from protoplasm simultaneously with the other 

 tissues ? These are profoundl}^ interesting questions ; but 

 questions to which positive answers cannot be expected. 

 All that can be done is to indicate answers which seem 

 feasible. 



That the property specially displayed by nerve, is a pro 

 perty which protoplasm possesses in a lower degree, is mani 

 fest. The sarcode of a Rhizopod and the substance of an 

 unimpregnated ovum, exhibit movements that imply a propa- 

 gation of stimulus from one part of the mass to another ; and 

 through the nerveless body of a polype, we see slowly 

 travelling and spreading a contraction set up by touching a 

 tentacle — a contraction which implies the passage from part to 

 part of some stimulus causing the contraction. We 



have not far to seek for a probable origin of this phenomenon. 

 There is good reason for ascribing it to the extreme insta- 

 bility of the organic colloids of which protoplasm consists. 

 These, in common with colloids in general, assume different 

 isomeric forms with great facility ; and they displaj^ not 



