DIFFERENTIALS IN FERTILIZATION 311 



5. There may be elimination of part of both maternal and paternal chromo- 

 somes and inhibition of development. Unfavorable. 

 Arbacia 9 X Toxopneustes $ (different suborders) 

 Toxopneustes 9 X Arbacia S : fairly compatible (different suborders) 



There is, here, at least an indication of a parallelism, although not a com- 

 plete one, between relationship and the results of heterofertilization. The 

 best results were obtained in the case of hybridization of suborders. In these 

 experiments we notice differences between the reactions in reciprocal hetero- 

 fertilizations. Differences in reciprocal hybridizations were noted also in 

 fertilization between different orders of echinoderms ( Echinocyanus and 

 Parechinus), when plutei, in some way intermediate between both parents, 

 could be obtained if certain combinations were used. Similar differences we 

 have observed also in the case of transplantation; even in this respect there 

 is thus a correspondence between transplantation and hybridization. 



Of special interest are the hybridizations between Drosophila melanogaster 

 and Drosophila simulans, because in these insects the genetic constitution of 

 the two parent species has been analyzed very carefully, primarily by genetic 

 methods, and more recently by a cytological study of the chromosomes. 

 Sturtevant found that in both these species, the second and X chromosomes 

 contain the same genes and that the latter are arranged in the same order, but 

 D. simulans has a long inversion in the right limb of chromosome 3, as com- 

 pared to melanogaster. Certain variations between these two species pertain 

 to the different distances between the genes in the corresponding chromosome, 

 also to the different lengths of the Y chromosomes and to the relative amounts 

 of heterochromatin in the X chromosomes. 



Examination of somatic cells in the hybrids between Drosophila melano- 

 gaster and Drosophila simulans suggests the possibility that differences in 

 certain genes prevent, here, the normal union of homoiogenous chromo- 

 somes. Even such slight differences in gene composition as exist between 

 these two species and the resulting incomplete union of chromosomes, lead 

 to sterility in the hybrid. Other species of Drosophila cannot be hybridized, 

 presumably because of the greater differences in gene constitution. If, how- 

 ever, crosses are made between still more nearly related organisms, as for 

 instance in the experiments of Lancefield, who hybridized two races of 

 Drosophila pseudoobscura, abnormalities of a lesser degree may arise, espe- 

 cially during the process of crossing over, but sterility results if in certain 

 chromosomal loci the alleles are derived from the two races. 



In the hybridization between species as nearly related as horse and donkey, 

 incompatibilities occur during meiosis in the male sex cells of the hybrid, and 

 in the primary spermatocytes of the F t generation abnormal mitoses appear. 

 A lack of coordination in the action of chromosomes derived from these dif- 

 ferent species leads to disturbances. Under certain conditions even lympho- 

 cytes may be attracted by the abnormal substances which are presumably 

 present at later stages of the development of tissues in such hybrids. 



Differences in the results of reciprocal fertilization are very evident in the 



