92 THE AMPHIOXUS. 



over-arcliings. These foldings and stretchings can be 

 explained through the mutual relation of the neigh- 

 bouring parts, through the pushing and pulling which 

 the latter exercise upon one another. What are the 

 powers however which call into play this pushing or 

 pulling ? The mechanical causes are of various sorts. 

 In the end they may all be referred to the activity 

 of protoplasm. We can however classify certain ap- 

 pearances according to common significations. 



First of all we can distinguish between processes 

 which are occasioned by contractions of protoplasm, 

 which can, in a stricter sense, be described as active 

 changes of form, and such processes as are to be 

 referred to growth. In this respect, the difference 

 between the energies of growth in neighbouring 

 portions plays an important part. Thereby foldings 

 as well as stretchings are effected. Many alterations, 

 as for instance growths, may be immediately referred 

 to the finer processes in protoplasm which are less 

 accessible to our understanding. 



The active changes of form are indeed confined to 

 short periods of development, where they often effect 

 rapid and important appearances of development. In 

 the section of development which lies before us we 



