Child, Driesch's harmonic equipotential systems in form-regulation. ßÜ 



least over an area equal to the whole length of the new primordium. 

 Consequently a primordium following a primordium has dispro- 

 portionately long distal parts as compared with a primordium 

 following a hydranth. 



The differences in this case are certainly not due to the deve- 

 lopment of primordia of different sizes in the perisarc-tube of 

 approximately the same size, neither are they due to stretching 

 of the stem proximal to the primordia removed, for at the time 

 of their removal the stretching had not begun. 



In these cases it is very clear that conditions or processes in 

 one part of the stem play a part in determining the character of 

 morphogenesis in another part. The proportions of the primordium 

 differ according to the conditions to which the stem has been sub- 

 jected before its isolation. In this case, then, external factors in 

 Driesch sense (See p. 579) are not and cannot be excluded, and 

 if they play a part in this case, there is every reason to suppose 

 they do in others. In short, these experiments show positively 

 that the proportions of the primordia in Tubularia are determined 

 in some degree by conditions external to them, and, what is also 

 important, conditions which cannot be excluded in any experiment. 



As to the use of mathematics in cases like that Tubularia, 

 I agree perfectly with Driesch (Driesch, 1908, p. 412). My measure- 

 ments of Tubularia were made for exactly the same purpose as his 

 own. I was of course well aware that Driesch did not regard 

 proportionality in Tubularia as mathematically exact: anyone who 

 examines a few primordia cannot fail to be convinced on this point. 

 On the other hand, Driesch did not measure all of the different 

 areas of the primordia separately, and he confined his measurements 

 to primordia from certain regions of the stem. Consequently his 

 measurements fail to show what mine show very clearly, viz., 

 a typical and practically constant difference in proportion between 

 primordia from different regions and poles. His data are simply 

 insufficient to extablish his conclusion that proportionality is approxi- 

 mately maintained in Tubularia, and more complete series of 

 measurements would have made such a conclusion impossible, unless 

 Driesch's idea of what constitutes approximate proportionality is 

 very different from my own. But in any case Tubularia does not 

 agree with the definitions of harmonic equipotential systems, which 

 have been quoted above. We do not find that „es steht hier näm- 

 lich „jeder" der möglichen Effekte zu jedem anderen in einem ganz 

 festen relativen Lage Verhältnis" (Driesch, 1899 a, p. 73). 



IV. Later definitions of harmonic equipotential systems. 



Driesch's later definitions of harmonic equipotential systems 

 seem to differ rather widely from those which have already been 



39* 



