ts mother. As the summer draws to a close each passes into a
resting-stage against the winter cold--the butterfly as a pupa and the
plant as a seed, with the difference that while the caterpillar provides
its own coat, that of the plant is provided by its mother. With the advent
of spring both butterfly and {5} plant emerge, become mature, and
themselves ripen germ cells which give rise to a new generation.

Whatever the details of development, one cardinal fact is clear. Except for
the relatively rare instances of parthenogenesis a new individual, whether
plant or animal, arises as the joint product of two sexual cells derived
from individuals of different sexes. Such sexual cells, whether ovules or
ova, spermatozoa or pollen grains, are known by the general term of
GAMETES, or marrying cells, and the individual formed by the fusion or
yoking together of two gametes is spoken of as a ZYGOTE. Since a zygote
arises from the yoking together of two separate gametes, the individual so
formed must be regarded throughout its life as a double structure in which
the components brought in by each of the gametes remain intimately fused in
a form of partnership. But when the zygote in its turn comes to form
gametes, the partnership is broken and the process is reversed. The
component parts of the dual structure are resolved, with the formation of a
set of single structures, the gametes.

The life cycle of a species from among the higher plants or animals may be
regarded as falling into three periods: (1) a period of isolation in the
form of gametes, each a living unit incapable of further development
without intimate association with another produced by the opposite sex; (2)
a period of association in which two gametes become yoked together into a
zygote and react upon one {6} another to give rise by a process of cell
division to what we ordinarily term an individual with all its various
attributes and properties; and (3) a period of dissociation when the single
structured gametes separate out from that portion of the double structured
zygote which constitutes its generative gland. What is the relation between
gamete and zygote, between zygote and gamete? how are the properties of the
zygote represented in the gamete, and in what manner are they distributed
from the one to the other?--these are questions which serve to indicate the
nature of the problem underlying the process of heredity.

Owing to their peculiar power of growth a