REACTION OF EMBRYONIC CELLS TO SOLIDS 541 



obtain in the center of tissue cultures after a few hours of incu- 

 bation has been proved by means of indicators. ^^ A given cell 

 would thus be exposed on one side to a more acid medium diffus- 

 ing from the tissue, and on the other to the more alkaline culture 

 medium. This condition would, in accordance with the Quincke- 

 Biitschli surface tension theory of amoeboid activity, bring about 

 the formation of pseudopodia on the side of the cell turned toward 

 the fresh medium and thus produce a centrifugal movement. 

 Similarly the flattening of an isolated cell or the spreading of a 

 sheet of cells on a smooth surface would be accounted for. Bur- 

 rows thinks that no movement takes place as the result of mere 

 contact with solids without the secondary stimulus resulting 

 from the chemical change. 



These considerations at least show that even under very simple 

 conditions, where no chemical stimuli have been intentionally 

 apphed, the local activities within a culture may nevertheless 

 give rise to such conditions as might stinmlate the cells chemi- 

 cally on one side and thus direct cell movement. It however 

 remains a fact that the chemical stimuli in question are pow- 

 erless to call forth these movements in the absence of solid sup- 

 port, else the large drop cultures would not behave as they 

 do. The acidity theory also offers no adequate explanation of 

 the adaptation of single cells to such minute structures as the 

 web fibers, nor of the fact that outwandering cells rapidly bridge 

 a gap between two separate pieces of tissue in the same culture. 

 While it must therefore be admitted that chemical stimuli may 

 play an important part in influencing the movements of cells 

 in simple cultures, as Burrows has pointed out, the facts show that 

 the cells are also stimulated by solids as such and respond to 

 them by an orienting movement. 



Response to tactile stimuli is of such general occurrence in 

 animals that there is nothing anomalous in the manifestation 

 of the same kind of sensitiveness in cells. Though in the metazoa 

 the responses are brought about by complex neuro-muscular 

 mechanisms, the reactions to mechanical stimuli given by tissue 

 cells and the protozoa are closely comparable with one another. 



28 Cf. Burrows, ibid, and Eous, Jour. Exp. Med., vol. 18, 1913. 



