466 C. G. MACARTHUR AND E. A. DOISY 



up of this substance or an interference with its rate of reaction. 

 Any variations in growth would be due to alterations in the 

 general growth produced by other substances or conditions. 



Though the hormones and the active principles in the internal 

 secretions are very popular these days, it seems rather too much 

 to expect that one and only one of them possesses such vita- 

 listic properties. It seems more rational to suppose that they are 

 active in bringing about alterations in growth, but that the main 

 process is independent of them. There is practically no evidence 

 that such substances are determiners of growth in unicellular 

 organisms. If one accepts the autocatalytic theory, it seems 

 necessary to give up the protoplasmic theory, for protoplasm, too, 

 should be simply a product and does not possess growing power. 

 As a result of this and other work, it can be stated with consid- 

 erable certainty that neither nucleus nor cytoplasm causes growth 

 to take place autocatalyticaUy, If one believes that the evidence 

 for the living, growing nature of the protoplasm as a whole is well 

 founded, chemical autocatalysis should be discarded. The data 

 agree so well with the theory, however, that there must be some 

 reason why a substance in a living organism, as well as the whole 

 tissue or organism should add largest absolute amounts of sub- 

 stance (table 13, fig. 2) during the middle of the growth period. 

 It is worthy of mention, though it is probably not a fundamental 

 explanation to say that protoplasm has an inherent power, when 

 unimpeded by the lack of food or too much of the products of its 

 activity, to increase in a geometrical ratio. As is well known, 

 bacteria and unicellular organisms increase in number and in 

 absolute weight (when retarding factors are small) in this 1, 2, 4, 

 8, 16 ratio. If such numbers are plotted against tune, the first 

 part of the S-shaped curve is obtained. The latter half of the 

 curve is produced through decreasing the geometrical ratio by 

 the retarding effect of lack of food or production of toxic products. 

 By analogy, such a curve should be produced in a multicellular 

 organism, through the division and development of the cells 

 producing it. It is thus seen that the essential characteristics of 

 autocatalysis are the necessary result of cell division in an im- 

 perfect environment. Of course, one of the reasons why cells do 



