Only those which quit the body retain
their power of living.
Of all those countless cells which, in the course of a lifetime, are
thrown off from the body, only one kind is adapted for existence
outside the body, namely, the reproductive cells.
Among the lower animals the reproductive cells often leave the body of
their parents only after the death of the latter. This is not the case
in man.
All the cell series which do not take part in the formation of
reproductive cells, as well as all the reproductive cells without
exception, or with only a few exceptions, die through unfavorable
external conditions; just as all, or almost all, of the infusoria
which arose from the double cell die before they can conjugate again.
At times, however, some of the infusoria persist till the next period
of conjugation, and in the same way, from time to time, some of the
human reproductive cells succeed in conjugating, and from them a new
individual arises.
A man is the outgrowth of the double cell produced from the
conjugation of two human reproductive cells, and consists of all the
cells which arise from this and remain in connection with each other.
The human individual originates at the moment of the mingling of the
nuclei of the reproductive cells; and the details of this mingling
determine his individual peculiarities.
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