INTRODUCTION 11 



been cut off from either the same individual or from a different individual, 

 succeeds when there is an autogenous relationship between the remaining part 

 of the cell and the pseudopod, but it leads to abnormal reactions when there 

 is a homoiogenous relationship. The nature of the reaction seems to depend 

 on a specific sensitive state of the ectoplasmic layer of the cell protoplasm, and 

 also in part on the diffusion of certain substances into the surrounding 

 medium. In various species of Paramecium peculiar agglutination reactions 

 between different individuals belonging to a certain species, have been 

 observed, which are characteristic of each species. In general, cells belonging 

 to the same group do not agglutinate with one another, but individuals be- 

 longing to well defined, strange groups of the same species do agglutinate. 

 These reactions resemble those of organismal differentials insofar as rela- 

 tionship between different organisms is a factor which determines the reac- 

 tion, but they differ from organismal differential reactions in that the reaction 

 seems to depend upon the condition of a restricted portion of the unicellular 

 organism and that specific functions are accomplished by means of these 

 reactions, which are those of certain organs rather than of organismal differ- 

 entials. A similar problem arises in regard to the relations between sperma- 

 tozoa and eggs. These relations are in certain respects comparable to those 

 existing between graft and host; but while in the latter an autogenous rela- 

 tionship is most adequate, in the case of sperm and ova a homoiogenous 

 relationship seems in many instances to be as good, or even better, than an 

 autogenous condition. Indeed, in some organisms, plants as well as animals, 

 specific mechanisms exist which tend to prevent autofertilization. These 

 mechanisms depend apparently upon the reaction which takes place, perhaps 

 by means of contact substances, between certain somatic cells belonging to the 

 female organism and the spermatozoa or its analogue in plants, or in other 

 cases they depend upon the direct interaction between egg and spermatozoon. 

 In the adult organism the various organs and tissues may possess, in addi- 

 tion to the typical species and individuality differentials, structures and sub- 

 stances which are specific not only for this particular organ and tissue, but 

 also for the species to which the organism belongs. The organs and tissues 

 of related species as a rule resemble one another more closely than those of 

 more distant species. The substances which are the bearers of these character- 

 istics may, therefore, have something in common with the species differential 

 or even with the individuality differential substances. However, they differ 

 from the latter in that they are peculiar to a certain organ or tissue. They 

 are not identical with the typical species differentials ; this is indicated also by 

 the fact that their chemical reactions may differ in certain respects from 

 those of the typical species or individuality differentials. We may designate 

 these characters and substances as secondary or accessory organismal differ- 

 entials. In many cases it is not possible to determine to which of these two 

 classes a certain substance belongs and then we must be content to apply the 

 term organismal differential, and in particular, species and individuality 

 differential, in a general way, comprising both the primary and secondary or 

 accessory organismal differentials. 



