240 



SCIENCE 



[Vol. LVI, No. 1444 



out in a centrifugal direction away from the 

 piece of tissue implanted; thus a more or less 

 radial and often tree-like branching arrange- 

 ment of the outgrowing cell strands is pro- 

 duced. We have discussed above the stereo- 

 tropic response. The centrifugal arrangement 

 can be conveniently studied in amoeboeyte tis- 

 sue. Several years ago we tested experiment- 

 ally the possible significance of the galvanic 

 current and differences in electric potential in 

 the centrifugal direction of cell migration. 

 Our results were negative, neither did the di- 

 rection of light rays influence the movements. 

 It is very probable that the centrifugal growth 

 depends upon the following two factors, (1) 

 the tendency of the cells adhering to each other 

 to separate, to send out pseudopodia and to 

 move in such a direction that two or more cells 

 forming a clump become isolated, (2) the ten- 

 dency of a 'healthy cell to continue for a cer- 

 tain time to form pseudopods at the same part 

 of the cell at which this process has been in- 

 duced. These two factors, the existence of 

 which observation of the moving cells verifies, 

 would be sufficient to produce the centrifugal 

 movement, which would thus partly represent 

 a statistical chance phenomenon. Inasmuch as 

 the isolated cells and the cell clumps become 

 less frequent with increasing distance from the 

 central piece, this would insure a centrifugal, 

 more or less net-like growth, or in other cases 

 a tree-like growth, such as we found under 

 various conditions. We may assume that the 

 same factors play a role during the growth 

 within the organism. 



Tissue formation depends upon a combina- 

 tion of cell movements and cell divisions. We 

 have seen that contact with a solid base is a 

 prerequisite for the outgrowth of tissues; but 

 in addition we found more recently that in 

 various tissues growing in vitro, a very active 

 mitotic cell division may be induced in favor- 

 able liquid culture media supplied with a suf- 

 ficient amount of oxygen, even under condi- 

 tions which do not permit an active outgrowth 

 because of the lack of a solid surface along 

 which the cells would be able to migrate.^" 



While it is thus possible to separate experi- 



i^iLeo Loeb and Moyer S. Fleisher, Journ. Med. 

 Besearch, 1919, SX, 509. 



mentally mitotic ceU proliferation and migra- 

 tion of cells in tissue cultures, the same stimu- 

 lus usually tends to produce both of these re- 

 actions together. A suitable change in en- 

 vironmental conditions usually brings about 

 alterations in the cells, which lead to both mi- 

 totic cell division and migration in tissues, 

 provided they are capable of both of these 

 activities. The difference in actual response 

 on the part of different tissues depends upon 

 the structure of the tissue, which enables cer- 

 tain kinds of cells to migrate very readily and 

 to divide only with difficulty or not at all, and 

 other tissues to divide much more readily than 

 to make amoeboid movements. 



The stereotropic reaction of tissue ceUs to 

 an environmental change consists of two com- 

 ponent parts, (a) of amoeboid movement and 

 (b) of alterations in the consistency of the out- 

 er layer of the tissue cells; the latter change 

 may vary very much in different cases. In some 

 cases a previously non-sticky surface may be- 

 come sticky as the result of the environmental 

 eihange (as in amosbocytes, the free surface of 

 endothelial cells, perhaps in lymphocytes and 

 certain other cells), while in other cases a cer- 

 tain degree of usually circumscribed adhesive- 

 ness pre-existed, but became more general or 

 stronger as the result of the environmental 

 change. 



These changes in motUity and adhesiveness 

 are responses of a living organism to a stimu- 

 lus. This is quite apparent in the case of the 

 amoeboeytes and in the reactions of the ex- 

 perimental amoeboeyte tissue, where a mechani- 

 cal stimulus leads to that far going alteration 

 in cell consistency, which makes possible both 

 stereotropic reactivity and tissue formation; 

 but in principle, conditions are similar also in 

 the case of other tissues. 



Leo Loeb 



Washington TJniveksitt 



THE SHENANDOAH CAVERNS 



The exhibition of caverns to the traveling 

 public is noted by the United States Geological 

 Survey as a growing industry in the Shenan- 

 doah Valley of Virginia. The famous Valley 

 Pike, now a link in the New York to Atlanta 

 highway, is traversed yearly by thousands of 

 automobile tourists, and no one has adequately 



