reproduction, the male sex-cell, which is scarcely more than a minute mass
of chromatin provided with a thin coat of protoplasm and a motile organ,
fuses with the egg, and the nuclei of the two cells unite to form a double
body, which contains equal contributions of chromatin from the two
parental organisms. This gives the physical basis for paternal inheritance
as well as for maternal inheritance, and it shows why they may be of the
same or equivalent degree. When, now, the egg divides, at the first and
later cleavages, the chromatin masses or chromosomes contained in the
double nucleus are split lengthwise and the twin portions separate to go
into the nuclei of the daughter-cells. As the same process seems to hold
for all the later divisions of the cleavage-cells whose products are
destined to be the various tissue elements of the adult body, it follows
that all tissue-cells would contain chromatin determinants derived equally
from the male and female parents. As of course only the germ-cells of an
adult organism pass on to form later generations, and as their content of
chromatin is derived not from the sister organs of the body, but from the
original fertilized egg, there is a direct stream of the germ plasm which
flows continuously from the germ-cell to germ-cell through succeeding
generations. It would seem, therefore, that the various organic systems
are, so to speak, sister products in embryonic origin. The reproductive
organs are not produced by the other parts of the body, but their cells
are the direct descendants of the common starting-point namely, the egg.
As the cells of the reproductive organs are the only ones that pass over
and into the next and later generations, it will be evident, in the first
place, that the germ plasm of their nuclei is the only essential substance
that connects parent and offspring. This stream of germ plasm passes on in
direct continuity through successive generations--from egg to the complete
adult, including its own germ-cells, through these to the next adult, with
its germ-cells, and so on and on as long as the species exists. It does
not flow circuitously from egg to adult and then to new germ-cells, but it
is direct and continuous, and apparently it cannot pick up any of the
body-changes of an acquired nature. Now we see why individual acquisitions
are not transmitted. The hereditary stream of germ plasm is already
constituted before an animal uses its parts in adult life; we