296 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



and female mycelia of certain fungi may behave in an analogous manner. 

 However, we have also pointed out some important differences between the 

 mechanisms underlying apparently analogous reactions in these primitive 

 organisms and in vertebrates. It is possible that not all the reactions in these 

 primitive organisms are of the same kind, and in some of those which we 

 have described the probability that individuality differentials are involved is 

 greater than in others. 



We may find even in some of these unicellular, apparently primitive organ- 

 isms, much finer differentiations between individuals than those which are 

 noted in relatively simple adult metazoan invertebrates, such as Hydra and 

 Planaria, or in the embryos of vertebrates. Although protozoa and the gametes 

 of algae belong to classes of organisms which are considered primitive, it 

 seems that within these classes there have developed, in the course of evolu- 

 tion, very fine differentiations between single cells, which cannot as yet be 

 observed in the organisms from which they are presumably derived. It is 

 therefore possible that in these classes of unicellular organisms mechanisms 

 or substances have evolved, in certain respects analogous to but probably not 

 identical with individuality differentials. The reactions which they manifest 

 are at least partly localized in the ectoplasmic structures of these cells ; but 

 inasmuch as the latter may be newly formed by the rest of the protoplasm in 

 a protozoon temporarily deprived of them, we must assume that also other 

 parts of these cells, including perhaps their nuclear substance, have the power 

 to give origin to their own specific individuality differential-like substances. 



As we have already mentioned, the observations discussed in this chapter 

 may be of significance also in the analysis of the conditions underlying the 

 formation of colonies which, in some instances, develop from unicellular 

 organisms. Individuality differential-like mechanisms in unicellular organisms 

 tend to keep the individuals separate from other individuals of the same species 

 and thus to insure to those organisms the maintenance of a separate existence. 

 Conversely, it may be concluded that whenever colony formation occurs, 

 reactions characteristic of individuality differential-like substances, such as 

 we have here described, are lacking. 



As to the interpretation of the mechanism underlying contact reactions be- 

 tween unicellular organisms, possessing their own individuality or species 

 differential-like mechanisms, Jensen and Verworn started with the assump- 

 tion that the protoplasm of these cells is liquid throughout. However, from 

 what has been learned since about the constitution of amoeboid cells in pro- 

 tozoa, in amoebocytes of Limulus, and even in cells of higher organisms, it 

 appears that the consistency of the ectoplasmic layer of isolated cells is gen- 

 erally more or less solid, although readily undergoing changes, and that under 

 different conditions its consistency may vary between the extremes of a com- 

 pletely solid and a liquid state. In the case of the organisms under discussion, 

 the contact between the surface layers of two unicellular animals or plants, 

 which latter differ in what in higher organisms would correspond to organis- 

 mal differentials, may, under certain conditions, act as an abnormal stimulus 

 initiating a softening of the surface layer; this change may be followed by 



