300 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



certain agencies, as for instance, acid dissolved in isotonic NaCl solution 

 within a certain range of concentration, not only diminish agglutination, but 

 may even cause a separation of agglutinated amoebocytes from one another 

 and thus change a tissue-like formation back into a suspension of isolated 

 cells. The agglutination and resulting tissue formation represent, therefore, 

 to a certain extent, reversible processes. A similar reversibility we find also 

 in some of the tissues of the most differentiated vertebrates. 



2. With amoebocyte tissue we can imitate and analyze certain phenomena 

 of wound healing which takes place in the normal epidermis of higher organ- 

 isms. Embedded in this tissue the amoebocytes are at rest, but as soon as an 

 incision is made and a piece cut out, the cells adjoining the wound become 

 active and migrate into the wound, thus tending to cover it. There is a differ- 

 ence in the environment of different parts of the cells adjoining the wound, 

 these cells being in contact with other amoebocytes, on the side away from the 

 wound and with a fluid medium and a glass surface on the side of the defect ; 

 and this condition acts as a stimulus, causing amoeboid movement in the 

 direction towards the wound and away from contact with the cells. Similarly, 

 we can excise small pieces of such a tissue, place them on a cover glass, and 

 treat them as we do pieces of higher tissues according to the tissue culture 

 method. Through secondary processes which, under certain conditions, may 

 become degenerative, the character of various higher tissues, especially those 

 of a mesenchymatous nature, may be imitated, and also pictures corresponding 

 to outgrowing fibroblastic tissue may be readily obtained with this experi- 

 mental amoebocyte tissue. The same factors which are responsible for the 

 movement into the wound of cells and groups of cells adjoining a defect, 

 cause also the active movement of cells in tissue culture. During this process 

 of migration the moving cells meet fresh amoebocytes which are likewise 

 migrating through the culture medium; if they come in contact with one 

 another, they stick together and form small clumps of cells, from which the 

 individual amoebocytes tend to detach themselves again; in this way cell 

 movement takes place, both in tissue culture and in wound healing, in a 

 centrifugal direction, similar to the cell behavior of higher vertebrate tissues 

 under analogous conditions. There is no indication that the movement is 

 otherwise an oriented one; on the contrary, we may consider it as more or 

 less a chance phenomenon. 



As stated, it is the physical and chemical changes in the environment which 

 bring about the agglutination of cells and, therefore, those reactions which 

 transform the cells from free-living, isolated cellular organisms into com- 

 ponents of tissues. If a corresponding condition existed within the blood 

 channels, as a result for instance of the introduction of a foreign body into 

 the blood, an agglutination would take place here also, which would lead to 

 the formation of an agglutination thrombus consisting of amoebocytes and 

 comparable to thrombus formation in higher organisms, where analogous 

 cells or blood platelets, representing parts of cells, furnish the substratum of 

 the thrombus. Tissue formation and thrombus formation are thus essentially 

 related processes. 



