HYBRIDIZATION AND TRANSPLANTATION 329 



death of the transplant. Similarly, after transplantation of cat ovary to rabbit 

 there was early degeneration, although at first there may still have been 

 noticeable some mitotic activity. 



As we have seen, in a general way a parallelism may be expected to exist 

 between the transplantability of tissues of certain organisms and the possi- 

 bility of hybridizing them, and, on the whole, the experiments of Schultz 

 indicate the actual existence of such a parallelism; but there are quite a num- 

 ber of exceptions to this rule and to some of them we have already drawn 

 attention. Thus reciprocal hybridizations may give different results and such 

 results may not correspond to those which are found in the case of corre- 

 sponding reciprocal transplantations. We have pointed out the differences 

 which exist in the significance of autogenous, syngenesious and homoiogenous 

 relationships in hybridization and in transplantation. While the results of 

 heterogenous relationships are more similar in hybridization and transplanta- 

 tion, a perfect correspondence is lacking even here. But a strict parallelism 

 should not be expected, because hybridization and transplantation, as pre- 

 ceding chapters have shown, represent in some very important respects very 

 dissimilar processes. 



To mention some of these differences : The chromosomes of horse and 

 donkey meet in the somatic cells of the mule without any apparent injury to 

 cells resulting from this heterogenous combination. On the other hand, skin 

 of the horse cannot be grafted successfully to the donkey, nor does the re- 

 ciprocal transplantation succeed. Furthermore, the fact that although hybrids 

 between two species may be well formed and strong, yet the eggs of the female 

 hybrid may not be fertilized by the spermatozoon of a male hybrid, can be 

 readily understood if we consider that the function of the male and female 

 chromosomes is not the same in the germ cells and in the specialized somatic 

 cells of the hybrids. The chromosomes of the germ cells undergo synapsis and 

 reduction divisions, which are very complex processes. Before reduction divi- 

 sion has taken place, the sex cells and the surrounding somatic cells have the 

 same set of genes, but they differ following this occurrence. During reduction 

 division in the hybrids, abnormalities may arise, which prevent the formation 

 of healthy spermatozoa and ova and thus lead to sterility. 



Schultz, in general, seems however to assume that the mutual interaction 

 of sex cells and of somatic cells is of the same kind, and that the germ cells 

 are more differentiated than the somatic cells, because they have the poten- 

 tiality of reproducing the whole organism. But early ontogenetic stages of 

 tissues do not yet show the same degree of differentiation of organismal 

 differentials as do adult tissues. Likewise, there is evidence for a phylogenetic 

 evolution of organismal differentials. Because homoio- and even heterotrans- 

 plantation may succeed in certain amphibia, it does not necessarily follow 

 that such transplantations must succeed also in mammalian organisms. 



If two animals of the same species differing in certain characteristics, as 

 for instance, in the pigmentation of certain parts of the skin, are mated, then 

 in the F 2 generation a segregation of these allelomorphs may take place. Two 

 individuals, A and B, belonging to the same litter, may therefore differ in the 

 color of a certain part of their skin. Schultz holds that it should be more 



