ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 345 



All these factors have some determining relation to the movements of 

 the cells. Tissues were taken from the embryos of Rana palustris just 

 after the closure of the medullary folds and from chick embryos. The 

 fluid media used were physiological sodium chloride, Locke's and Ringer's 

 solutions in varying degrees of concentration, the blood plasma of the 

 frog, and the serum of the chicken. The means used to support the 

 planted tissue were the fibrin network of the clotted plasma, spider web, 

 and the surface of the cover-glass. The use of pieces of spider web 

 proved a very effective innovation. 



The experiments lead to the conclusion that solid objects are an 

 important and even necessary factor in .the movement of embryonic 

 cells, such as mesenchyme and epithelium. Leaving out the cases of 

 movement upon the surface film, there are no exceptions to the rule 

 that movement takes place only when contact with solid material is 

 attained. Each of the three kinds of solid support used in the experi- 

 ments influenced the cell movement in its own way. 



The cpiestion arises whether these reactions are to be regarded as a 

 manifestation of stereotropism (ihigmotaxis), which is a response to 

 mechanical stimulation (pressure), or whether the solid acts only in- 

 directly by inducing conditions that give rise to chemical or s6me other 

 form of stimulation. Burrows has shown that the centrifugal movement 

 of cells observed in almost all cultures, i.e. the movement from the 

 implanted cell mass out into the culture medium, may be explained by 

 the acidity produced in the main mass of cells through the accumulation 

 of waste products. Pseudopodia will be formed on the side away from 

 the acidity. But Harrison points out that the chemical stimuli are 

 powerless to call forth the movements in the absence of solid support. 

 Moreover, the acidity theory offers no adequate explanation of the 

 adaptation of single cells to such minute structures as the web fibres, 

 nor of the fact that outwandering cells rapidly bridge a gap between two 

 separate pieces of tissue in the same culture. The facts show that the 

 cells are stimulated by solids as such, and respond to them by an orienting 

 movement. Harrison finds no reason for refusing to call the reaction 

 to solids a tropism. 



It may be that stereotropism may have somethiug to do with the 

 phenomena of normal development, e.g. of the sheath cells of an em- 

 bryonic nerve. Similarly, the close application of mesenchyme cells to 

 such structures as blood vessels, muscles, and various other organs, 

 resulting in the formation of a cellular sheath, which afterwards becomes 

 sclerotized, may be due in the first instance to a stereotropic response. 

 The surface of structures such as the medullary cord, notochord, ali- 

 mentary canal, muscle-plates and the inner surface of the epidermis 

 would serve as a solid base upon which cells may creep. In the encyst- 

 ment of foreign bodies within an organism a similar phenomenon is 

 "liserved. 



With regard to the movements of the growing nerve fibre, the evi- 

 dence points again to the conclusion that the protoplasm is stereotropic. 

 No free outgrowth of nerves in a fluid medium has ever been observed, 

 But such solids as a flbrin clot or a smooth glass surface readily serve to 

 support them. And so may the surfaces of the larger cell masses and 



