Modern researches have shown that the nucleus, when the cell divides,
assumes the form of a spindle of fibres, associated with which are
distinct bodies called chromosomes, that the number of these chromosomes
where it can be counted is constant for all individuals of the same
species, and that before the gametes are ready for fertilisation two
cell-divisions take place, which result in the reduction of the number of
chromosomes to half the original number. When two gametes unite, the
specific number is restored. Since the male gamete is very small and seems
to contribute to the zygote almost nothing except the chromosomes, which
carry with them all the characters of the male parent, it seems a
necessary conclusion that the chromosomes alone determine the character of
the adult. There are, however, facts which point to an opposite
conclusion.

Hegner, [Footnote: R. W. Hegner, 'Experiments with Chrysomelid Beetles,'
III., _Biological Bulletin_, vol. xx. 1910-11.] for example, found that in
the egg of the beetle _Leptinotarsa_, which is an elongated oval in shape,
there is at the posterior end in the superficial cytoplasm a disc-shaped
mass of darkly staining granules, while the fertilised nucleus is in the
middle of the egg. When the protoplasm containing these granules was
killed with a hot needle, development in some cases took place and an
embryo was formed, but the embryo contained no germ cells. Here no injury
had been done to the zygote nucleus, but these particular granules and the
portion of protoplasm containing them were necessary for the formation of
germ cells. In other experiments a large amount of protoplasm at the
posterior end of the ovum was killed before the nucleus had begun to
segment, and the result was the development of an embryo consisting of the
head and part of the thorax, while the rest was wanting. The nucleus
segmented and migrated into that part of the superficial cytoplasm which