318 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



thus makes self-fertilization possible. There are also other methods which 

 have such an effect ; the most interesting one of these is, perhaps, the induction 

 of polyploidy by means of colchicine treatment of some branches in Petunia 

 axillaris (Stout and Chandler). Self-compatibility was procured for all seed- 

 lings obtained from the self- fertilized flowers of tetraploid branches. More- 

 over, these seedlings could be cross-fertilized ; likewise, backcrosses to parents 

 were fertile, except the combination of a tetraploid female seedling and a 

 diploid male parent. The genetic balance is presumably changed in these 

 tetraploid plants in such a way that the mechanism controlling the movement 

 of the pollen-tube towards the ovule is no longer inhibited. How many genes 

 are involved in this process in polyploid organs is not known. 



If we compare conditions in transplantation of mammalian tissues with 

 those in pollination experiments, we find certain analogies. The results in 

 both processes depend upon whether certain substances are the same or are 

 different in the two interacting cells or cell complexes. This determines the 

 relations between host and transplant, as well as those between spermatozoon 

 and female sex apparatus. In both cases the reactions are more intense in 

 a homoiogenous than in an autogenous or syngenesious relationship, but 

 homoiogenous reactions injure a transplant, whereas they are as a rule bene- 

 ficial in pollination. Furthermore, in transplantation we noticed in certain 

 instances marked differences in the types of reaction resulting from a reversal 

 of the relationship between transplant and host, and in a similar way, dif- 

 ferences were observed in certain plants in the case of reciprocal fertilization. 

 There is another observation which is of special interest because it corresponds 

 to certain findings in transplantation. If the same plant is pollinated by two 

 different types of pollen, each one behaves in its own way, uninfluenced by 

 the presence of the other. In a similar way we have found that in case of simul- 

 taneous transplantation of pieces of tissues from two different donors into 

 the same host, the specific reactions, as determined by the mutual relation- 

 ships between the individuality differentials of the various transplants and 

 the host, take place around each transplant in their characteristic manner, 

 without any influence of the other transplant being noticeable. 



However, there are also important differences between transplantation and 

 pollination. While in transplantation an autogenous condition is the most 

 favorable one for a satisfactory union between host and graft, in fertilization 

 identity of the specific substances which come into play is in many cases 

 unfavorable for the production of a fertilized ovum. There is an additional 

 difference in that in the former all degrees of gradations in the results occur, 

 whereas, in fertilization we find either compatibility or non-compatibility, the 

 latter leading to sterility; no inter-grades exist as far as the end result con- 

 sisting in the fertilization of a single ovum by a spermatozoon is concerned. 

 In the pollination process, itself, it is nevertheless possible to recognize certain 

 gradations in the degree of compatibility between the male and female cells, 

 as is indicated by the varying rapidity of the downgrowth of the pollen-tube, 

 and in some cases, by the number of eggs which are fertilized and of embryos 

 which develop in a normal manner. 



