male and female, is needed to start the amazingly complex process of
building a new individual. Thus in flowers the stamens, the pollen
bearers, provide the male element which, through the intermediary of
the pistils, fertilizes the egg in the vesicle. In the higher animals
the egg or ovum is produced by the female, and is fertilized by the
sperm-cell produced by the male. The necessary union between these two
essential elements is attained in various ways. Thus the female salmon
deposits her eggs on a convenient spot in the bed of a stream and the
attendant male salmon then projects over them the spermatozoa. In the
higher animals there is a further development, and special organs are
evolved to ensure the conjunction of the two elements. I have not space
to describe in detail the effect of this union of the two cells,
generally spoken of as fertilization. It may be found fully recorded
step by step in any biological manual. Very briefly, the sperm-cells,
which are active, freely moving units, swarm round the egg-cell and one
of them eventually enters it. The essential part of the cells, namely
the nuclei, coalesce into one nucleus, and an active process of cell
division and multiplication is at once started. The single cell divides
into two daughter cells, then again into four, and so on. Very early in
development, the cells, which at first appear similar, become
differentiated into different types, but the whole ordered sequence of
the development of an embryo is achieved by this cell division and
multiplication. Each original cell contains a substance which, on
account of its being easily colorable with artificial stains, is called
chromatin, and this chromatin is believed to be the bearer of the
hereditary qualities. The cell division is so arranged that each new
cell receives an equal share of the male and female chromatin, and this
process is continued in every case of cell division, so that
eventually, in every part of our bodies, the dual inheritance remains
complete.