104 SCIENCE AND PHILOSOPHY OF THE ORGANISM 



physiological functions of plants : the roots are seen to bend 

 away from the light and towards the ground ; the branches 

 behave just in the opposite way. It now has been supposed 

 by Herbst that such " directive stimuli ' may also be at 

 work among the growing or wandering parts of the embryo, 

 that their growth or their migration may be determined by 

 the typical character of other parts, and that real morpho- 

 genetic characters can be the result of some such relation ; 

 a sort of " chemotropism " or " chemotaxis " may be at work 

 here. Herbst himself has discussed theoretically several 

 cases of organogenesis in which the action of directive 

 stimuli is very probable. What has become actually 

 known by experiment is not very much at present : the 

 mesenchyme cells of Echinus are directed in their migration 

 by specified places in the ectoderm, the pigment cells of the 

 yolk-sac of the fish fundulus are attracted by its blood 

 vessels, and nerves may be forced to turn into little tubes 

 containing brain substance ; but of course only the first two 

 instances have any bearing on typical morphogenesis. 



The first case of an " internal formative stimulus ' in 

 the proper sense, that is, of one embryonic part causing 

 another to appear, was discovered by Herbst himself. The 

 arms of the so-called pluteus of the sea-urchin are in 

 formative dependence on the skeleton no skeleton, no 

 arms ; so many skeleton primordia, 1 in abnormal cases, so 

 many arms ; abnormal position of the skeleton, abnormal 

 position of the arms : these three experimental observa- 

 tions form the proof of this niorphogenetic relation. 



1 I use the word "primordia" for the German " Aulage" ; it is better than the 

 word "rudiment," as the latter may also serve to signify the very last stage 

 of a certain formation that is disappearing (phylogenetically). 



