OF THE GERM CELLS OF METAZOA. 213 



ing generations, there should be an even number in the spermatogonia and ovogonia. The 

 uneven number discovered in the four species mentioned may have arisen in one of two 

 ways : through bastardization, or through a mitotic abnormality, each of which possibilities 

 may now be considered. 



In the case of bastardization of a germ cell with one chromosomal number by a 

 germ cell from another species with a different number, the uneven number eleven might 

 be secured if a paternal germ cell of a species A, with the normal chromosomal number 

 twelve, fertilizes a maternal germ cell of a species B with the normal number ten. In 

 species A the reduction in number of the chromosomes would give six univalent chromo- 

 somes in the spermatid, and in species B five univalent chromosomes in the matured 

 ovum ; conjugation of the two cells would then result in 6 + 5 = 11 univalent chromo- 

 somes. In this way an uneven chromosomal number may have arisen by the conjugation 

 of the germ cells of species with different numbers of chromosomes. But the objection to 

 this view lies in the fact that hybridization of distinct species is generally infertile, and 

 species which would have different chromosomal numbers would probably be quite distinct. 



More probably, then, the uneven number may have originated through abnormalities 

 of mitosis, and there would be many possibilities for such an occurrence. (1) It may have 

 arisen in a spermatogonic mitosis, in a species where the ancestral normal number is 

 twelve, by the chromatin of the spirem segregating abnormally into only eleven chromo- 

 somes, so that one of these chromosomes would be virtually bivalent. This would seem 

 to have been the origin of the large odd chromosome x of Protenor be/fragci, which ap- 

 pears bipartite in the spermatogonia and also in the spermatocytes, though in the latter it 

 does not conjugate with any other chromosome. Such a case would be a deficiency in the 

 segregation of the chromatin in the prophases of mitosis. (2) Or the uneven chromo- 

 somal number may have arisen by an unequal distribution of the chromosomes in the 

 anaphases of mitosis, so that the daughter cells would not receive equal numbers. 



Now, whether the uneven chromosomal number had originated through bastardiza- 

 tion, or, what would be more probable, through abnormality in mitosis, it is interesting to 

 determine how this number can perpetuate itself through different generations of the 

 species, for my observations show that in the species where it is found it occurs in all 

 individuals. The following table gives in condensed form the mode of reduction of the 

 chromosomal number and valence in the four species in question, the chromatiu nucleoli 

 being omitted for the sake of simplicity : 



