228 EMBRYOLOGY 



that there is a progressive determination of structure. As a result the parts 

 of the older embryo self-differentiate in some detail when they are trans- 

 planted. They are not able to form new or different structures. The progres- 

 sive restriction of the potencies of cells if continued into the adult stage 

 would result in an individual in which each cell was restricted to a certain 

 definite structure and a certain function. No cell could then replace another 

 type of cell. Such a rigid determination of cells does not appear to be the 

 case since studies on regeneration on the adult show that cells which are 

 removed are replaced by other cells. In the next chapter we shall discuss the 

 principles of the process of regeneration. 



