314 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



ismal differentials. However, in these embryonal transplantations it is again 

 the autogenous relationship between host and graft which, on the whole, 

 results most readily in a harmonious combination, although in many cases 

 no distinct difference between an autogenous and a homoiogenous relation- 

 ship can be noticed. In certain respects the analogy between fertilization and 

 transplantation between single cells, or parts of single cells, such as the graft- 

 ing of pseudopods in protozoa, would seem to be greater than that between 

 fertilization and transplantation of tissues in higher organisms, but again, 

 comparisons show a very striking difference, insofar as here, also, only an 

 autogenous relationship between cells or parts of cells leads to a satisfactory 

 union, while, as stated, the homoiogenous relationship is, as a rule, the 

 normal one in fertilization. 



In addition to other similarities between transplantation and fertilization 

 there is the observation that a reciprocal relationship between the individuals 

 or species serving as host and transplant, and between male and female 

 gametes, may lead to very different results ; the latter are due to the differ- 

 ences which exist in these processes between the function of the host and the 

 ovum, on the one hand, and the donor tissue and spermatozoon, on the other 

 hand. In both these processes the phylogenetic relationship between the 

 interacting cells or tissues is a factor which helps to determine the outcome, 

 but the parallelism between such a relationship and the results is, on the 

 whole, much less evident in fertilization than in transplantation. 



Moreover, the reactions between the sex cells depend on very special 

 mechanisms which regulate the interaction between spermatozoon and egg, 

 and which are lacking in other primitive cells. Such mechanisms consist in 

 the specific functioning of the chromosomes of male and female sex cells, in 

 the relation of chromosomes to each other and to the cytoplasm of the ovum 

 and in the interaction of certain genes. As a rule, these interactions are 

 optimal in case of homoiofertilization and there may even exist in certain 

 instances means for preventing autofertilization. However, as in transplan- 

 tation, so also in fertilization, heterogenous relationships between the chromo- 

 somes of two parent strains lead to incompatibilities, which are greater if 

 distant species, than if nearly related species, or different races of the same 

 species, are combined. 



