EXPERIMENTAL MORPHOGENESIS 133 



in organogenesis ? We, of course, have to look back to our 

 well-studied " formative stimuli." These stimuli, be they 

 " external " or " internal," come from without with respect to 

 the elementary organ in which any sort of differentiation, 

 and therefore of localisation, occurs : but in our harmonious 

 systems no localising stimulus comes from without, as was 

 the case, for instance, in the formation of the lens of the 

 eye in response to the optical vesicle touching the skin. 

 We know absolutely that it is so, not to speak of the self- 

 evident fact that the general " means ' of organogenesis 

 have no localising value at all. 1 



So we see there is nothing to be done, either with 

 the means or with the formative stimuli ; both are entirely 

 unable to account for those kinds of localisation during 

 differentiation which appear in our harmonious systems. 



But is there no possibility of explaining the phenomena 

 of organogenetic localisation by auy other sort of interaction 

 of parts ? Two such possibilities may at the first glance 

 seem to exist. 



1 One might object here that in a piece of a Tubularia stem, for instance, 

 the tissues are in direct contact with the sea-water at the two points of the 

 wounds only, and that at these very points a stimulus might be set up say 

 by a process of diffusion which gradually decreases in intensity on its way 

 inward. And a similar argument might apply to the small but whole 

 blastula of Echinus, and to all other cases. But, in the first place, stimuli 

 which only differ in intensity could hardly call forth the typical and typically 

 localised single features realised in differeutiatiou. On the other hand 

 and this will overthrow such an hypothesis completely the dependence of 

 the single localised effects in every case on the absolute size of the frag- 

 ment or piece chosen for restoration renders quite impossible the assumption 

 that all the singularities in the differentiation of the harmonious systems 

 might be called forth by single stimuli originating in two fixed places in an 

 independent way. These would never result in any "harmonious," any 

 proportionate structure, but a structure of the "normal" proportionality 

 a/id size at its two ends and non-existent in the middle ! 



