es and man, there is no
reproduction without conjugation; no parthenogenesis or budding. So
far as we have studied the question we see in the animal and vegetable
kingdoms sexual reproduction, or conjugation, as a _sine qua non_ for
the indefinite continuation of life.

=The Sexual Glands. The Embryo.= However complicated the organism, it
always possesses a special organ, the cells of which, all of the same
form, are reserved for the reproduction of the species and especially
for conjugation. The cells of these organs, called _sexual glands_,
have the power of reproducing themselves so that they reconstruct the
whole individual (the type of the species) from which they arose, in
an almost identical form, by conjugation (sometimes also, for a
certain time, by parthenogenesis) under certain fixed conditions as
soon as they leave its body. We can thus say with _Weismann_, speaking
philosophically, that these germinal cells continue the life of their
parents, so that in reality death only destroys part of the
individual, namely, that which has been specially adapted for certain
exclusively individual ends. Each individual, therefore, continues to
live in his descendants.

The germinal cell divides into a number of cells called embryonic,
which become differentiated into layers or groups which later on form
the different organs of the body. The embryonic period is the name
given to the period between the exit of the germinal cell from the
maternal body and the final complete development which it acquires in
becoming the adult individual. During this period the organism
undergoes the most singular metamorphoses. In certain cases it forms a
free embryo which appears to be complete, having a special form and
mode of life, but which finally becomes transformed into an entirely
different sexual individual. Thus from the egg of a butterfly there
first emerges a caterpillar, which lives and grows for some time, then
changes to a chrysalis and finally to a butterfly. The caterpillar and
the chrysalis belong to the embryonic period. During this period every
animal reproduces in an abbreviated manner certain forms which
resemble more or less those through which its ancestors have passed.
The caterpillar, for example, resembles the worm which is the ancestor
of the insects. _Haeckel_ calls this the _fundamental biogenetic law_.
We are not concerned here with embryology, and will content ourselves
with some of the main points.

=Ger