540 ROSS G. HARRISON 



in the large drop showed the same condition. Two of the former 

 showed cell movement, i.e., some wandering upon the cover 

 and the web fibers but no nerves were present. In all of the 

 large drop preparations (fig. 14) the contour of the tissue was 

 relatively smooth and no cells wandered out. These experi- 

 ments tend to corroborate the results obtained with frog tissues, 

 that the web fibers are not favorable to the outgrowth of nerves, 

 though they are too few in number to be at all conclusive in this 

 respect. 



DISCUSSION 



The foregoing experiments lead to the conclusion that solid 

 objects are an important and even necessary factor in the move- 

 ment of embryonic cells, such as mesenchjane and epithelium. 

 Leaving out the cases of movement upon the surface fihn, which, 

 moreover, has certain properties of solids, there are no exceptions 

 to the rule that movement takes place only when contact with 

 solid material is attained. Furthermore, each of the three kinds 

 of solid support used in the experiments influences the cell move- 

 ment in its own way, as is shown by the different arrangement 

 assumed by the cells after a certain period of cultivation (figs. 

 8 to 10). The reactions to solids take place whether the fluid 

 medimn is a saline solution or sermn, though the latter is conduc- 

 ive to more active movement (and growth) than the former, owing 

 no doubt to its nutritive qualities.-^ 



The question now arises whether these reactions are to be re- 

 garded as a manifestation of stereotropism (thigmotaxis), which 

 is a response to mechanical stimulation (pressure), or whether 

 the solid acts only indirectly by inducing conditions that give 

 rise to chemical or some other form of stimulation. Biu-rows" 

 has shown that the centrifugal movement of cells observed in 

 almost all cultures, i.e., the movement from the implanted cell 

 niass out into the culture medium, may be explained by the 

 acidity produced in the main mass of cells through the accumu- 

 lation of waste products. That a condition of acidity does 



2« Cf. M. R. and W. H. Lewis, Anat. Rec, vol. 6, 1912. 

 27 Proc. Am. Cong. Phys. and Surg., 1913. 



