38 CYTOMORPHOSIS 



also the daily multiplication of cells which have remained 

 embryonic. 



We recognize that the embryonic cells are of great impor- 

 tance not only during the embryonic period, but also in the 

 adult. How great this importance is is revealed in the inves- 

 tigation of regeneration. Very many animals, if parts of 

 their body are removed, will form the missing parts anew. If, 

 for example, we break off the tip of the tail of a lizard, there 

 will arise a new tip which is formed by the growth of undif- 

 ferentiated tissues. There are worms which multiply by 

 forming in the middle of their bodies the so-called budding 

 zone. Karl Semper^^ has studied the process in annelids, 

 and discovered that in them the budding zone consists of cells 

 of the embryonic t)^e. Gradually these cells advance in 

 their cytomorphosis, and so there arises a new tail for the 

 anterior part of the worm and a new head for the posterior 

 part, and thereupon the two parts separate and two complete 

 works have arisen from the single animal. We are accustomed 

 to designate those animals which have a more complicated 

 structure as the higher. Now it is clear that if an animal is 

 composed of relatively few cells great complexity of structure 

 is impossible. Further we observe that when a highly formed 

 animal is to be produced, nature takes care that a large 

 number of embryonic cells is produced. In the lower animals 

 development is of the so-called larval type. From the little 

 ovum there arises quickly a young animal which lives free 

 and must take care of itself. Such a larva must possess, even 

 if only in simple form, all the principal organs, and since the 

 cells must be so far differentiated that they can take over the 

 various functions, they necessarily lose in part their capacity 

 to multiply, and, what is still more important, the capacity 

 to produce other kinds of cells. We see always that when 



