Chapter 2 

 Tissue Formation and Organismal Differentials 



We have seen that incompatibilities between the organismal differ- 

 entials or their precursors, or between substances analogous to 

 these differentials, but not identical with them, which are present 

 in adjoining cells may prevent the union of the latter and lead to the separa- 

 tion of cells or parts of cells at or near the point where the bearers of the 

 incompatible differentials come in contact. This applies to the union of ova 

 and of embryos, or parts of embryos, in very early stages of development, 

 as well as to the union of free-living, unicellular organisms or parts of them. 

 In other cases it may merely modify the nature of their union. There is a 

 related phenomenon of great biological interest, namely, the formation of 

 tissues through the union of single cells. Here apparently similar factors to 

 those which we have discussed in the preceding chapters are active and it may 

 therefore perhaps be possible to analyze the conditions on which the union of 

 various cells into tissues depends, and to determine whether there is any 

 indication that in this process, also, organismal differentials or related sub- 

 stances play a part. 



1. A very simple and primitive type of tissue results from the agglutina- 

 tion of amoebocytes of Limulus, which takes place spontaneously whenever 

 the blood of this animal leaves the body under natural conditions. Because of 

 the primitive nature of this process, it exemplifies, perhaps, some of the prin- 

 ciples underlying tissue formation in general, and moreover, it is more readily 

 accessible to experimental analysis than the more complex processes leading 

 to the formation of the fixed tissues in organisms. In contrast to the latter, 

 the amoebocyte tissue is merely an experimental tissue, but the analysis of the 

 factors underlying its formation has served as the starting point for similar 

 studies in the case of the more complex natural tissues. 



The essential factor underlying the formation of this amoebocyte tissue is 

 an agglutination process, and the agglutination is due to a change in the 

 environment of these cells, which acts as a stimulus. The stronger the stimulus 

 within a certain range, the greater are the changes in the amoebocytes and the 

 more intense is the agglutination which takes place. Thus, if we make an 

 incision into a Limulus and allow the blood to flow out through such a narrow 

 opening, it will come in contact with the rough surface of the wound and 

 subsequently with the chitinous body covering; under these conditions the 

 amoebocytes send out pseudopods and some of the cells may even change into 

 a diffuse gelatinous material. If the altered cells and the material flowing out 

 from the injured amoebocytes come in contact with one another they stick 

 together, so that they form one jelly-like mass, which gradually retracts into 

 a small firm clot, in this respect behaving therefore not unlike a blood coagu- 

 lum. But if, instead of using this simple process, we collect the blood by means 



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