LIMITS OF DIVISIBILITY OF LIVING MATTER. 65 



gastrula or a Pluteus. We find somewhat similar phenomena 

 in the processes of regeneration. A piece of the stem of a 

 tubularian hydroid is able to reproduce roots as well as polyps. 

 But it was shown by Miss Bickford in my laboratory, that if 

 the piece cut out of the stem is below a certain size it cannot 

 reproduce all the organs, but only one, namely, the polyp. 



But from all this it is clear, too, that the ultimate unit of 

 living matter, in a given species, is not a definite quantity of 

 matter, but that the quantity varies with the functions that we 

 use as a criterion for living matter. If we require that living 

 matter show phenomena of growth and organization, it must 

 consist of protoplasm plus nuclear substance. If we are satis- 

 fied with spontaneity or irritability, the ultimate unit is much 

 smaller and qualitatively simpler, since for these manifestations 

 the protoplasm alone is sufficient. However, the units of living 

 matter not only represent a definite quantity of energy, but we 

 have reason to assume that the setting free of this energy is 

 connected with a certain scheme, which may perhaps be the 

 same for all the phenomena of life. If such is the case our 

 further knowledge of the ultimate elements is likely to be 

 dependent upon our understanding of this scheme. A full 

 knowledge of this or these schemes would be a solution of the 

 riddle of life. 



