80 CARNEGIE INSTITUTION OF WASHINGTON. 



played by the surface stickiness or adhesiveness which the cells exhibit for 

 certain solids and for each other. Although we have been vaguely aware that 

 many cells are sticky, the great importance of this adhesive quality in the 

 various body-cells has been almost entirely ignored. It is now shown by Dr. 

 Lewis that it is due to this quality that the form of most of our tissues and 

 organs is maintained, and that, were it absent, there would be nothing to 

 prevent our bodies from disintegrating. Without it, a multicellular organism 

 could not exist. I shall speak later of his observations on the so-called syncy- 

 tial relations supposed to exist in many tissues, in which he found evidence 

 that these relations consist merely of adhesion of cell processes. In tissue- 

 cultures he shows that migrating cells are so sticky for glass that they can be 

 washed or centrifugalized without disturbing their attachment. He has thus 

 far been unable to determine whether this quality is dependent upon the com- 

 position of the protoplasmic surface or upon some sort of substance that is 

 secreted by the cell. In support of the latter theory there is the demon- 

 strable occurrence around some cells of a cement substance that can be recog- 

 nized by its power to reduce silver nitrate. He finds that most living cells in 

 tissue-cultures exhibit a slight browning over their entire surface when treated 

 with silver nitrate, while dead cells do not. This, however, may mean that 

 silver nitrate is reduced by living protoplasm and not by any substance on its 

 surface. It seems likely that the degree of stickiness may vary under dif- 

 ferent conditions. It may prove possible to measure the tenacity of the 

 adhesion by the centrifugal force necessary to dislodge the cells, but so far 

 even the high speed of the ordinary centrifuge fails to detach them. This 

 property of cells to adhere to solids appears to explain why migrating cells in 

 tissue- cultures follow solid supports, and it is probably erroneous to desig- 

 nate the phenomenon a tropism, as we have heretofore been inclined to do. 



Nature of Mesenchymal Reticulum. 



From the behavior of embryonic mesenchyme in tissue-cultures, Dr. Lewis 

 has shown that there is no valid evidence for the view that it is syncytial in 

 structure — that there is an actual fusion of the processes and transferal of 

 material from one cell to another. On the contraiy, the evidence indicates 

 that it is an adherent reticulum and that the cell processes merely stick to each 

 other by reason of the adhesive quality which he has shown they possess. 

 From a mesenchymal explant presenting the typical picture of a supposed 

 syncytium, cells can be seen to migrate out and completely isolate themselves 

 from the reticulum of which they were a part. In this process one can watch 

 the slow shifting of cells from one position to another and follow the with- 

 drawal of processes which were adherent or in contact with the processes or 

 bodies of the neighboring cells. The phenomenon of withdrawal and the 

 formation of new processes is the same as when the processes adhere to the 

 cover-glass, and there is never any evidence of tearing or rupture. Dr. Lewis 

 has further shown that in cultures that have been washed in certain solutions 

 the mesenchyme cells withdraw their processes and lose connection with the 

 neighboring cells. Such retraction may proceed until all connections with the 

 cover-glass are lost and the rounded cells fall to the lower surface of the drop. 

 If, however, the cells retain their attachment to the cover-glass they may 

 later send out new processes and a reticulum be again established. Some- 



