238 



SCIENCE 



[Vol. LVI, No. 1444 



be indnced and modified experimentally in the 

 amosboeytes and that it depends upon changes 

 in the consistency of the protoplasm, which 

 tend to make it sticky. Stereotropism is there- 

 fore an expression of the same factor which 

 leads to the agglutination of isolated cells with 

 each other, which latter results in tissue forma- 

 tion. 



The stereotropism of tissue cells depends 

 upon their ability to carry out amoeboid move- 

 ments. On the basis of our observations and 

 experiments we concluded that in amoeboid 

 movements, cyclic changes in the consistency 

 of those parts of the protoplasm take place 

 wM<!h are concerned in this activity.* With 

 these changes there must be associated cor- 

 responding changes in the agglutinative power 

 of the tissue cells. The factors underlying 

 amoeboid movements are therefore closely re- 

 lated to those determining agglutination and 

 tissue formation; but while in the latter usual- 

 ly the whole surface of the cell is in a viscous 

 state, the cyclic changes in consistency and 

 viscosity of the protoplasm are localized and 

 thus lead to the formation of pseudopods. In 

 modifying this amoeboid movement experiment- 

 ally we have shown ithat its character varies 

 with the consistency of the protoplasm; only 

 if the latter approaches the liquid state can 

 surface tension play a part in the amoeboid 

 movement; but during certain phases of the 

 process the consistency of the protoplasm is 

 so great that the laws of surface tension, as 

 they have been defined for the liquid state of 

 matter, cannot play an essential part in 

 amoeboid movement.'' 



Related to amoeboid movement but not 

 identical with it, is the spreading out of tissue 

 cells, which occurs under certain conditions, 

 when ithe cells are in contact with solid sur- 

 faces. This process can likewise very con- 

 veniently be studied in amoebocytes. Both 

 amoeboid movement and spreading out depend 

 upon changes in the consistency of the proto- 

 plasm. The spreading out in particular de- 

 pends upon a softening and relaxation of the 

 outer layer of the protoplasm. Usually this 

 spreading out goes hand in hand with amoeboid 



movement; yet both processes can to a certain 

 extent be separated from each other experi- 

 mentally. As we have shown, it is possible to 

 increase the consistency of the protoplasm in 

 such a way, that the spreading out is prevent- 

 ed or very much delayed, while amosboid move- 

 ments are still quite active. Addition of defi- 

 nite amounts of acids to the medium in which 

 ithe amcebocytes are held, may have this effect. 

 However, in influencing the consistency of the 

 outer layer of the protoplasm as a whole, for 

 instance through the addition of acid, we may 

 also influence the character of the pseudopods 

 in a corresponding manner. Addition of slight 

 amounts of alkali favors very much the ex- 

 tension of the cells by softening the consistency 

 generally.* Again the consistency of the outer 

 layer of the spreading out cell is such tha,t 

 adhesiveness results; thus an agglutination of 

 the spreading out cells to the base on which 

 they rest is assured. This spreading out is an 

 important factor in tissue formation and dur- 

 ing embryonic development ithe formation of 

 the mesenchyme deipends upon such a trans- 

 formation of the more or less rounded off 

 blastomeres into cells, which not only assume 

 amceboid movement, but at the same time 

 spread out. We have previously called atten- 

 tion to the importance of changes in aggluti- 

 native properties of cells during embryonic 

 development, in which a change takes place 

 from agglutinated round cells, which stick to 

 each other with certain parts of their circum- 

 ference, to cells spreading out in contact with 

 a solid or viscous substratum.' 



It can be shown that the stickiness and 

 therefore the agglutinability of cells is a vari- 

 able factor; but the degree of this variability 

 differs very much in different kinds of cells 

 and under different conditions. We have 

 found that this variability is very great in the 

 ease of amoebocytes. In the circulating blood 

 they are not sticky; but as soon as the environ- 

 ment is altered they become sticky, show 

 amoeboid movements and agglutinate to each 

 other and to other structures. We have every 

 reason to believe that, while .these variations 

 in stickiness are less marked in the majority 



^Journal Med. Eesearch, 1902, VII, 145. 

 'Washington University Studies, 1920, VIII, 3. 



^Ameriean Journal Physiol. 1922, LX, 277. 

 sScffiNCE, 1922, LV, No. 1410. 



