312 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



experiments of Montalenti, who crossed Bufo viridis and Bufo vulgaris. In 

 this case the combination Bufo viridis ? X Bufo vulgaris $ was much more 

 unfavorable than the reciprocal combination, Bufo vulgaris 9 X Bufo 

 viridis $ . In the former type of hybrids retardation in development and 

 abnormal morphogenesis may affect early cleavages and gastrulations ; sub- 

 sequently, malformations appear, especially in the development of the ner- 

 vous system and of the heart. In the reciprocal crosses, alterations in early 

 stages of development, resulting in the death of the embryos, are very rare. 

 The large majority of these embryos develop like the controls, although at 

 first there may be some delay in development. The tadpoles seem to be about 

 normal, but later on, during metamorphosis, there is a considerable mortality. 



This difference in the results of reciprocal hybridization, as in those of 

 transplantation, may be attributed to the dissimilar role which host and donor 

 play in these processes ; in hybridization it is the ovum which acts as host to 

 the spermatozoon which it receives into its body. The dissimilarities in the 

 significance of egg and spermatozoon may be taken to indicate that it is not 

 only the chromosomes of these two cells which interact with each other, but 

 that the chromosomes of the male germ cells interact also with the cytoplasm 

 of the egg. 



In accordance with the more complex and delicate chemical differentiation 

 of cells and tissues, which progressively takes place during the development 

 of the embryo, the interaction of the chromosomes derived from the two 

 parents evidently becomes, correspondingly, a process of increasing delicacy, 

 and finer differences in the relationship between sperm and ovum may there- 

 fore, as a general rule, manifest themselves only during the later develop- 

 mental periods, while coarser differences may result in abnormalities at 

 much earlier embryonal stages. 



It appears that in some of these incompatibilities processes of a purely 

 mechanical character may be involved, as, for instance, maladjustment in the 

 size and shape of male and female chromosomes, or in the movements of the 

 asters and chromosomes, and, somewhat later, disturbances in the rhythms 

 of mitotic divisions and in the developmental rhythms characteristic of the 

 paternal and maternal species in general may interfere. However, there is 

 reason for believing that in hybridization interactions of a chemical nature 

 between substances derived from the male and female germ cells may also be of 

 importance; and in this respect, again, the mechanisms active in transplanta- 

 tion and in fertilization would then resemble each other. These chemical 

 interactions may be of a toxic nature, if the two individuals from which the 

 germ cells are derived do not belong to the same species ; toxic effects of this 

 kind have been suggested by Jacques Loeb and Moenkhaus. But it seems that 

 in some cases, in which the distance in relationship between the male and 

 female cells is very slight, the offspring may not only not be defective, but 

 embryonal development may, on the contrary, be accelerated ; observations 

 of this kind we shall discuss in the next chapter. Also in the hybrids, Bufo 

 vulgaris 9 X Bufo viridis $ , to which we have referred above, it was found 

 by Montalenti that there were some tadpoles, a few weeks old, which ac- 



