388 



ON INCREASE IN SIZE 



[PT. Ill 



organism is exactly the same in all four cases, but the manner in 

 which the increase in weight has taken place is in the four cases 

 profoundly different. It is certainly quite clear that the chemical 

 embryologist, engaged in the attempt to understand the processes 

 which contribute to the final result, must pay detailed attention to 

 the path by which this final 

 result is arrived at. The four 

 different curves in Ostwald's 

 figure would imply four very 

 different sets of conditions with- 

 in the developing embryo. An 

 embryo which grew according 

 to Curve I would change very 

 rapidly in the beginning, and 

 afterwards change progressively 

 less rapidly as the curve became 

 asymptotic. The reverse of this 

 process would happen if the em- 

 bryo grew according to Curve ii, 

 for there the process continually increases in rapidity, and is pro- 

 ceeding at its fastest when the point A is reached. Curve iii, on the 

 other hand, being S-shaped, would seem to indicate the presence of 

 an autocatalytic process, for at first the growth proceeds faster and 

 faster, but later on, after the point of symmetry of the curve has 

 been reached and passed, slower and slower. Several such S-shaped 

 curves superimposed on one another make up Curve iv. 



As far as is known, no growth takes place in the manner repre- 

 sented by Curve i, but rather in that of the other three curves, 

 though our present knowledge does not enable us to say definitely 

 which, except in certain cases. Ostwald's monograph should be 

 referred to by those who wish to see how he continued the discussion 

 of growth-curves, for it is probably the best presentation of the subject, 

 and it was certainly written from a much less doctrinaire point of view 

 than most of its successors. 



The general interpretations of embryonic growth-curves may be 

 divided into several classes. They depend more than anything else, 

 as will be clearly seen, upon how the facts are expressed. One way 

 of expressing them led Minot to his "laws of cytomorphosis", 

 another led Robertson to his "autocatalytic master-reaction", 



