UNION OF FREE-LIVING CELLS 295 



als exist. It would be of interest to compare, from this point of view, the 

 protoplasmic reactions in different colony- and plasmodia-forming organisms 

 with the corresponding reactions in types of organisms which live as isolated 

 individuals. 



We have seen that genetic relationship may determine not only the char- 

 acter of contact reactions, but may control also the movements of two organ- 

 isms which are at some distance from each other. Analogous reactions occur 

 likewise in cells of metazoa. The cytotropic reactions described by Roux 

 (1895) may possibly be of a similar nature. Roux found that cells of morulae 

 or blastulae of Rana, when separated from each other at no greater distance 

 than the diameter of a cell, may send out processes and move towards each 

 other. However, in this case reactions between homoiogenous cells were found 

 to be apparently of the same character as autogenous reactions. Cell move- 

 ments, which probably depend upon substances active at a distance, seem to 

 play a role also in embryonal development. For instance, in urodele larvae 

 certain mesoderm cells are attracted by and move towards the developing eye 

 vesicle ; in this case the organismal differentials have not yet reached a stage 

 of marked specificity, and accordingly a lack of individual specificity is noted 

 in the movements of these embryonal cells towards a transplanted eye vesicle. 

 As we have seen, in higher organisms the organismal differentials regulating 

 the interaction between cells and tissues are more finely graded. In addition 

 to the examples discussed already, we may mention the following observation 

 recorded by A. Fischer. If parts of two homoiogenous chick embryo hearts are 

 combined in vitro, cellular anastomoses between contractile elements of such 

 fragments are produced and synchronous pulsations of the two parts take 

 place ; but this reaction does not occur if the embryonal heart fragments, 

 placed in contact with each other under otherwise the same conditions, belong 

 to two different avian species. In a like manner, as we have already pointed 

 out, there is reason for attributing to autogenous morphogenic contact sub- 

 stances the function of maintaining in mammals the normal inter-relation and 

 balance between the different tissues of the same organism, the autogenous 

 tissue equilibrium. 



It seems then that reactions occur in unicellular organisms, which in some 

 respects correspond to those which, in higher organisms, we attribute to 

 organismal and especially also to individuality differentials. But as we have 

 seen, the same criteria as to organismal differentials which we applied in 

 higher organisms cannot, in a strict sense, be used in protozoa and unicellular 

 plants, because by definition the organismal differentials, and in particular the 

 individuality differentials, are substances present in all, or almost all of the 

 cells and tissues of a given individual and differentiate this individual from 

 all other individuals. It is clear that in a unicellular organism such a defini- 

 tion cannot apply. Still, we may at least conclude that certain protozoa and 

 flagellated gametes of algae possess structures and substances which function 

 in a somewhat similar manner to the organismal differentials of higher organ- 

 isms, inasmuch as they determine in a graded manner the reactions of these 

 cells to other cells in accordance with the genetic relationships. Also, the male 



