veri in
1902.)

The recently discovered fact that in diploid nuclei the chromosomes are
arranged in pairs affords additional evidence in favour of the unequal
value of the chromosomes. This is still more striking in the case of
chromosomes of different sizes. It has been shown that in the first
division-figure in the nucleus of the fertilised egg the chromosomes of
corresponding size form pairs. They appear with this arrangement in all
subsequent nuclear divisions in the diploid generation. The longitudinal
fissions of the chromosomes provide for the unaltered preservation
of this condition. In the reduction nucleus of the gonotokonts the
homologous chromosomes being near together need not seek out one
another; they are ready to form gemini. The next stage is their
separation to the haploid daughter-nuclei, which have resulted from the
reduction process.

Peculiar phenomena in the reduction nucleus accompany the formation of
gemini in both organic kingdoms. (This has been shown more particularly
by the work of L. Guignard, M. Mottier, J.B. Farmer, C.B. Wilson, V.
Hacker and more recently by V. Gregoire and his pupil C.A. Allen, by the
researches conducted in the Bonn Botanical Institute, and by A. and
K.E. Schreiner.) Probably for the purpose of entering into most
intimate relation, the pairs are stretched to long threads in which the
chromomeres come to lie opposite one another. (C.A. Allen, A. and K.E.
Schreiner, and Strasburger.) It seems probable that these are homologous
chromomeres, and that the pairs afterwards unite for a short time, so
that an exchange of hereditary units is rendered possible. (H. de Vries
and Strasburger.) This cannot be actually seen, but certain facts of
heredity point to the conclusion that this occurs. It follows from
these phenomena that any exchange which may be effected must be one of
homologous carriers of hereditary units only. These units continue to
form exchangeable segments after they have undergone unequal changes;
they then constitute allelotropic pairs. We may thus calculate what sum
of possible combinations the exchange of homologous hereditary units
between the pairing chromosomes provides for before the reduction
division and the subsequent distribution of paternal and maternal
chromosomes in the haploid daughter-nuclei. These nuclei then transmit
their characters to the sexual cells, the conjugation of which in
fertilization again produces the most varied combinations. (A