chromosomes form the part of
the cell which contain the hereditary traits handed down from parent to
child. This follows from the fact that the chromosomes are the only part
of the cell which, in the fertilized egg, is derived from both parents.
Now the offspring can certainly inherit from each parent, and hence the
hereditary traits must be associated with some part of the cell which is
derived from both. But the egg substance is derived from the mother
alone; the centrosome, at least in some cases and perhaps in all, is
derived only from the father, while the chromosomes are derived from
_both_ parents. Hence it follows that the hereditary traits must be
particularly associated with the chromosomes.

With this understanding we can, at least, in part understand the purpose
of fertilization. As we shall see later, it is very necessary in the
building of the living machine for each individual to inherit characters
from more than one individual. This is necessary to produce the numerous
variations which contribute to the construction of the machine. For this
purpose there has been developed the process of sexual union of
reproductive cells, which introduces into the offspring chromatic
material from _two_ parents. But if the two reproductive cells should
unite at once the number of chromosomes would be doubled in each
generation, and hence be constantly increasing. To prevent this the
polar cells are cast out, which reduces the amount of chromatic
material. The union of the two pronucleii is plainly to produce a
nucleus which shall contain chromosomes, and hence hereditary traits
from each parent and the subsequent splitting of these chromosomes and
the separation of the two halves into daughter nucleii insures that all
the nucleii, and hence all cells of the adult, shall possess hereditary
traits derived from both parents. Thus it comes that, even in the adult,
every body cell is made up of chromosomes from each parent, and may
hence inherit characters from each.

The cell of an animal thus consists of three somewhat distinct but
active parts--the cell substance, the chromosomes, and the centrosome.
Of these the cell substance appears to be handed down from the mother;
the centrosome comes, at least in some cases, from the father, and the
chromosomes from both parents. It is not yet certain, however, whether
the centrosome is a constant part of the cell. In some cells it cannot
yet be found, and there are some reasons