REACTION OF EMBRYONIC CELLS TO SOLIDS 543 



stimulus is not placed at a distance from the organism, for it is 

 the relative motion of the organism stimulated and the stimu- 

 lating object acting only when they come into contact, that 

 gives rise to the stimulus, and in the case of a moving organism 

 coming into contact with a fixed object the energy is supplied by 

 the organism itself. However this is not sufficient ground for draw- 

 ing general distinction between the response to such stimuli and 

 the reaction to a beam of light. The difference Hes only in the 

 source of the stimulus itself, i.e., in the distance from which it 

 comes, and in the continuousness of its effect. In the case of a 

 negative reaction to pressure the stimulus must cease as soon as 

 the recoil is made. In case of a positive reaction the stimulus 

 may continue but there can be no further visible effect beyond 

 the change in shape of the organism unless its parts are differenti- 

 ally sensitive. The similarity to phototropism becomes closer 

 in the case of rheotropism or reaction to currents in the sur- 

 rounding medium, which, as Verworn pointed out,='- is to be classed 

 under the group of responses to mechanical stimuli. 



A word remains to be said regarding the significance of the 

 facts brought out in this study with reference to the interpretation 

 of some of the phenomena of normal development. The re- 

 semblance between the arrangement of cells on the web fibers 

 (especially as seen in figure 12) and the sheath cells of an embry- 

 onic nerve suggests that stereotropism may have something to do 

 with the latter, though not necessarily to the exclusion of chemo- 

 tropic influences. Similarly the close application of mesenchjnue 

 cells to such structures as blood vessels, muscles, and various 

 other organs, resulting in the formation of a cellular sheath, 

 which afterwards becomes skleratized, may be due in the first 

 instance to a stereotropic response. In fact the conditions with- 

 in the embryo at the time when the tissues are taken for the 

 experiments are such as to suggest that this stereotropic reaction 

 is an .important factor in the behavior of many kinds of cells. 

 There is a very active movement of mesenchyme at the time, 

 during which cells stream froi^i certain regions and fill in the 



'2 Allgemeine Physiologie. 



