INTRODUCTION 21 



To return to the individuality in the structure and chemical constitution of 

 the egg, the conceptions of Fick and others evidently do not localize these 

 characteristics in the nuclear genes, but in the cytoplasm. There can be no 

 doubt at the present time as to the significance of the chromosomes and genes, 

 or other subdivisions of chromosomes, and of the arrangement of the latter 

 in the chromosomes, for the determination of species and individual characters 

 although differences of interpretation exist as to the mode of their repre- 

 sentation in the chromosome. There is further no agreement, as yet, among 

 investigators as to the importance which must be attached to other factors 

 in addition to the chromosomes. We have referred already to the views of 

 Correns, von Wettstein and Kuhn, who assume that also the cytoplasm 

 carries genes which determine development ; this conclusion was based on the 

 results of species hybridizations in which reciprocal combinations gave differ- 

 ent results and in which an influence of the maternal germ cells was noticeable, 

 in accordance with the views of Jacques Loeb (1916), who had restricted the 

 Mendelian mode of heredity to certain individual characteristcs of organs 

 and tissues, while he believed that species characteristics are determined by 

 the cytoplasmic structure of the ovum. In a similar way von Wettstein assumes 

 that the hereditary substance which is localized in plasma differs in its signifi- 

 cance from that of the genes of the chromosomes. He suggests that it is the 

 former which is the real substratum of the developmental processes, whereas 

 chromosomal genes and environmental factors control and direct the processes 

 which are dependent upon the structure of the cytoplasm. 



However, the majority of geneticists at the present time hold that the genes 

 are the substratum which determines the hereditary transmission not only of 

 the individual but also of the species, genus and class characteristics from 

 generation to generation, and that the genes impress upon the cytoplasm of 

 the ovum, by their interaction with the latter, the structure which is specific 

 for each species. We would, accordingly, have to assume that the individual, 

 species, genus, order and class differentials, in general the organismal differ- 

 entials, are preformed in both egg and spermatozoa and that, thus, precursors 

 of the organismal differentials exist in these germ cells, the nature of which 

 is determined by the genes of the egg and spermatozoon. However, whether 

 all the genes participate equally in the determination of the organismal differ- 

 entials, or whether some of the genes predominate in this function over others, 

 is not known. Inasmuch as no difference has been found between the male and 

 female sex in regard to the transmission and possession of the organismal 

 differentials or their precursors, it may be assumed that the Y chromosome, 

 which is concerned with the sex differentiation of the fertilized egg, does not 

 play an essential role in the determination of the constitution of these pre- 

 cursors. This would accord with the indications which exist that the Y chromo- 

 some does not as a rule carry demonstrable alleles of sexlinked genes, at least 

 not many of them. This interpretation of the nature of the precursors of the 

 organismal differentials is the most probable one, because egg and spermato- 

 zoon, as far as we know, contribute equally to the constitution of these pre- 

 cursors. In regard to the organ and tissue differentials, however, which 

 characterize and distinguish the various organs and tissues in the same indi- 



