he groups of chromosomes, the rays disappear and the
cell divides into two halves, each containing a group of chromosomes
(Fig. 9); the indentation increases so as to form a partition across
the protoplasm. The chromosomes then form a new meshwork of nuclear
chromatin, and we have then two cells each with a nucleus and a
centrosome like the mother cell (Fig. 10).

This is what takes place in the reproduction of all cells of the
animal and vegetable kingdoms. In the simplest unicellular organisms
which are known fission constitutes the only means of reproduction. In
the complicated organisms of the higher plants and animals each cell
divides in the manner indicated above, both in the embryonic period
and later on during the development of each of the organs which forms
the organism. This fact shows more than any other the intimate
relationship which connects all living organisms. The most remarkable
thing, perhaps, is the almost mathematical division of the
chromosomes into two halves, a division which results in the equal
distribution of their substance through the whole organism. We shall
return to this point later on.

=Reproduction by Budding. Parthenogenesis.= In the animal and
vegetable kingdoms the higher organisms become more and more
complicated. They are no longer composed of a single cell, but of an
increasing number of these cells combined in a whole, of which each
part, adapted for a special purpose, is itself formed of cells,
differentiated as much by their organic form as by their chemical and
physical constitution. In this way, in plants, are formed the leaves,
flowers, buds, branches, trunk, bark, etc.; and in animals the skin,
intestine, glands, blood, muscles, nerves, brain, sense organs, etc.
In spite of the great complication of the divers living multicellular
organisms, one often finds among them the power of reproduction by
fission or by budding. In certain animals and plants, groups of cells
vegetate in buds which separate from the body later on and form a new
individual; this occurs among the polypi and plants with bulbs, etc.
One can even form a tree by means of a cutting. Ants and bees, which
have not been fecundated, are capable of laying eggs which develop by
_parthenogenesis_ (virgin parturition) and become complete
individuals. But these degenerate and disappear if reproduction by
parthenogenesis or budding is continued during several generations.

Among the higher animals, the vertebrat