September 1, 1922] 



SCIENCE 



239 



of cells, great variations occur generally. Thus 

 in the resting stage, the endothelial cells lining 

 blood and lymph vessels present a perfectly 

 smooth surface and other cells pass them free- 

 ly without adhering to them. But when cer- 

 tain changes in the environment occur, these 

 cells may become amoeboid and migrate ad- 

 hering to surfaces; or they assume phagocytic 

 properties, which likewise presuppose certain 

 changes in .the surface consistency. It is very 

 probable that similar changes occur in all 

 epithelial or connective tissue cells, when they 

 change from the resting state to the active 

 state of amoeboid migration, adhering to sur- 

 faces with which they come in contact. 

 Amoeboid movement as such implies, as we 

 have shown, a cyclic, localized change in the 

 consistency of the cell and it can be shown 

 that the stickiness of .the pseudopod may be 

 greater than that of the rest of the cell. In 

 the majority of the cells the variation in con- 

 sistency and stickiness is less marked than in 

 the case of the amoeibocytes, because they are 

 already naturally in the state of tissue com- 

 ponents, adhering with certain parts of their 

 surface to neighboring cells. This applies al- 

 ready to the blastomeres of the dividing egg, 

 in which some parts of the outer layer have 

 such a degree of viscosity that the segmenting 

 cells remain united. The union of cells in 

 resting tissues may be accomplished by means 

 of secondary differentiations, which lead to the 

 production of special structures. There is 

 reason for believing that when, under the in- 

 fluence of certain stimuli these cells enter into 

 the state of amreboid movement, these differen- 

 tiations are at least partially lost and this im- 

 plies a change in the condition of the outer 

 layer of the cell, which exhibits amoeboid 

 movement and on which the agglutinability, 

 and therefore stereotropism, depends. 



Such changes in environment which lead to 

 amoeboid movement, alterations in consistency 

 of protoplasm, stereotropic response, play a 

 great part in pathological condi.tions, in in- 

 flammation, wound healing, and tumor growth, 

 in all of whi<*h cellular stereotropic move- 

 ments are a significant factor. Under natural 

 conditions these movements occur not only 

 during embryonic development in various types 



of cells, but also in the adult organism, for 

 instance in the ovary during atresia of the 

 follicles, in the endothelial cells of the corpus 

 luteum during the formation of blood vessels 

 and in the lymphocytes migrating through 

 epithelial surfaces. 



It is possible further to analyse some of the 

 factors and especially the significance of ag- 

 glutination in the formation of certain tissue 

 structures. In general, cell division not ac- 

 companied by amoeboid movements tends to the 

 production of epithelial surfaces. Tissues 

 which show active amceboid and spreading out 

 movements, may produce structures which dif- 

 fer according to the relative degree of agglu- 

 tinability and energy of amoeboid movement 

 present. If the amoeboid motility is very 

 marked in proportion to the degree of ad- 

 hesiveness, we tend to have tissues of a more 

 or less loose character. Thus the fibroblasts 

 are connected with each other through rela- 

 tively small areas of contact, while they are 

 sending out long pseudopodia and moving free- 

 ly along solid bodies. In epithelial tissues, or 

 in tissues of similar morphological character, 

 the preponderance of amoeboid motility over 

 the adhesiveness of the cells is much less pro- 

 nounced and these cells are therefore joined 

 together in wider surfaces of contact and may 

 form more or less connected layers. The same 

 differences we find between the growth of sar- 

 coma cells, which behave similarly to fibroblasts, 

 and carcinoma cells which are epithelial in 

 character. Great stickiness may lead to the 

 formation of cell clumps, and if it is associated 

 with a tendency to amoeboid movement and 

 spreading out of the adhesive cells, it may lead 

 to the foi-mation of tissue layers, in which 

 strands of spread out cells form a net. This 

 net formation is due to the fact that the cells 

 move away from the cell clumps, which serve 

 as centers, and form thus secondarily strands 

 of spread out cells, which connect with each 

 other. 



In the process of tissue formation, such as 

 we observe in tissue growing in culture media 

 in vitro, two phenomena are noticeable in out- 

 growing cells, (1) the stereotropism, which 

 leads the cells to grow in contact with solid 

 surfaces, (2) the tendency of the cells to grow 



