DIFFERENTIALS IN FERTILIZATION 313 



quired a larger size, underwent metamorphosis earlier and exhibited lower 

 mortality than the controls. In such instances of heterofertilization conditions 

 apparently exist comparable to those characteristic of homoiogenous fertili- 

 zation ; but, in addition, certain differences between the germ cells may exert 

 a stimulating effect which is favorable instead of being injurious. 



While, then, within certain limits there is presumably a proportionality 

 between the incompatibilities which develop in hybridization and the distance 

 in relationship between egg and spermatozoon, exceptions to this rule do 

 occur and may be very striking. They are most likely due to the presence of 

 secondary factors superimposed upon the primary ones, which latter would 

 act in accordance with the greater nearness or distance of relationship be- 

 tween spermatozoon and egg. Thus the degree of resistance to injurious con- 

 ditions on the part of these cells belonging to two different species may vary 

 in different cases, irrespective of phylogenetic factors. 



There is another fact indicating the lack of complete correlation which 

 may be present, in certain respects, between readily effected hybridizations in 

 different species and the phylogenetic relationship between egg and spermato- 

 zoon. It seems that in certain cases heterogenous fertilization may succeed 

 as well in teleosts as in echinoderms, although the latter stand much lower 

 in the phylogenetic scale than the former. Conditions are different in trans- 

 plantations ; here we find, as a general rule, that transplantability becomes 

 more and more restricted with increasing phylogenetic development. In 

 plants, on the other hand, transplantations seem to succeed over a wider 

 range of phylogenetic relationships than hybridizations. 



It may then be concluded that while there exist distinct similarities be- 

 tween transplantation and fertilization, there are also notable differences. To 

 recapitulate some essential facts : In both these processes we have to deal with 

 what may be considered a host-donor relation, the ovum representing the 

 host and the spermatozoon the donor cell in the case of fertilization. In both 

 processes the host has a function which differs from that of the donor and in 

 both the reactions of the host preponderate ; however, on the whole the con- 

 stitution and function of the spermatozoon are of a relatively greater sig- 

 nificance for the fate of the host, in the case of fertilization, than is the piece 

 of grafted tissue or organ for the recipient organism in the case of trans- 

 plantation. While in transplantation in adult mammals it is the character of 

 the organismal differentials in host and donor which is the most important 

 factor determining the fate of the transplant, in the interaction between 

 ovum and spermatozoon we have to deal with the precursors of organismal 

 differentials ; while in transplantation it is the autogenous relationship between 

 the organismal differentials which is most favorable for a satisfactory inter- 

 action between transplant and host, in fertilization it is the homoiogenous 

 relationship which may be considered normal and, as a rule, conducive to the 

 best results. But, if we compare fertilization with transplantation between 

 embryonal organisms, the difference between these two processes is less 

 pronounced, inasmuch as also in embryonal grafting we have to deal with 

 precursors of organismal differentials and not with fully developed organ- 



