398 ORGANIC GROWTH sec. 



of incomplete rudiments of fingers after fcetal self-amputation 

 in man.^ 



The reason of this is that the cells have still in the young 

 state — the younger, the more — the same general character as 

 in lower organisms; further, that the protoplasm has not 

 been completely used up in forming definite structures, 

 and is in every respect less exhausted, more capable of 

 life and growth than in the adult condition. The more 

 minute explanation of the differences will be given in what 

 follows. 



In organisms also which have no fixed external form, as, 

 for instance, many sponges, the recrescence of the whole out 

 of any part is not very surprising, and yet it is only a step 

 from this to the recrescence of the definite recfular forms of 

 such multicellular animals as the Hydra. How^ever, the 

 differentiation of the cell -layers even in sponges makes 

 the explanation of the process less self-evident. The 

 greater the differentiation of function and the constancy 

 and complexity of structure the more difficult it becomes to 

 understand recrescence. For from the formation of the f^erm- 

 layers onwards the functions of the cells of multicellular 

 animals increasingly diverge : the cells of the various layers 

 soon begin, if I may so express it, to meet with different 

 experiences, they must acquire different capabilities. They 

 must therefore gradually be restricted to the power of forming 

 only organs or parts of organs which proceed from the germ- 

 layer to which they belong. 



It is obvious, however, that in considering the question our 

 inquiries cannot stop at the germ-layers, the simple or primi- 

 tive organs which retain their original character in zoophytes 

 throughout life : they must be extended to every portion of a 

 germ-layer which is destined with others to form some part of 

 the body, and therefore to every complex organ, and in plants 



^ Cf. Spiegelberg, Lehrhuch der Geburtshiilfe, Lahr, 1878, p. 356. 



