308 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



organismal differential, identical in various organs and tissues of the same 

 individual and species. 



In many instances where, in plants and animals, both male and female 

 germ cells are produced in the same organism, mechanisms of a special kind 

 have developed, tending to prevent auto fertilization, which otherwise would 

 have been the simplest mode of fertilization but which might have injurious 

 consequences. Even syngenesio-fertilization occurring in succession through 

 many generations leads in many cases to a gradual deterioration of the 

 organism. 



On the other hand, if heterofertilization takes place, incompatibilities also 

 develop, as a rule, sooner or later, even if spermatozoon and egg belong to 

 relatively nearly related species; but these incompatibilities may in certain 

 respects be less marked than those in heterotransplantation and the spermato- 

 zoon may even, under such conditions, remain alive and apparently unharmed 

 in the strange ovum, whereas, after transplantation of differentiated tissues 

 between the corresponding two species in mammals the host reacts very 

 strongly against the transplant, which is severely injured and, as a rule, 

 destroyed within a relatively short time. Thus in Echinoderms, by means of 

 fertilization between different orders it is possible to produce hybrid plutei, 

 which in certain of their characteristics are intermediate between the two 

 parent orders. In some instances, a slight increase in the constitutional differ- 

 ences between spermatozoon and egg above those characteristic of the average 

 homoiogenous relationship between the organisms which carry the sex cells, 

 may even have a stimulating effect on the developmental processes resulting 

 from fertilization and may thus prove favorable at least in the first generation. 

 But in the case of transplantation the incompatibilities between transplant 

 and host, and the resulting injury of the transplant, increase rapidly with 

 increasing strangeness of the organismal differentials. 



However, notwithstanding these differences between fertilization and 

 transplantation, there is one very essential similarity; after heterogenous 

 fertilization as well as after heterogenous transplantation incompatibilities 

 do, as a rule, develop, which to a certain extent are the greater, the greater 

 the differences in the constitution of the organismal differentials or of their 

 precursors in the cells or tissues which are joined together. A markedly heterog- 

 enous character of the precursor substances of the organismal differentials 

 in egg and spermatozoon is associated with an abnormal interaction, causing 

 an interference with the development of the resulting hybrid; but with less 

 incompatible precursors of organismal differentials the development may 

 continue long enough for a specific organismal differential to form in the 

 hybrid, which thus acquires its own mechanism of reaction against strange 

 organismal differentials. 



As to incompatibilities developing between spermatozoa and ova, which 

 are sufficiently distant genetically from each other, these, in general, may be 

 caused by two factors: (a) an incompatibility between the spermatozoon 

 and the surface layer of the strange ovum; (b) incompatibilities between the 

 nuclei of these two cells, and in particular between their chromosomes, or 



