Some Problems of Reproduction: a Comparative Study of ...

76 MABCUS M. HAETOG.
4. Isogamy, plural or binary, is a step in advance of plasmodium
formation, involving, as well as plastogamy, KARYO-
GAMY, or the reconstitution of a nucleus by the fusion of. old
ones.
5. The rejuvenescence of karyogamy is due to the fact
that the zygote nucleus and cytoplast form a new cell
association.
6. A similar rejuvenescence may take place by the mere
migration of a nucleus into a vacant foreign cytoplast, as in
the union of a spermatozoon with the non-nucleated fragment
of the egg of an Echinoderm.
7. Many cases of so-called "parthenogenesis" involve
really the fusion of nuclei, the resulting nucleus being essentially
different from the fission nuclei of the previous cellcycle.
8. Other modes of rejuvenescence may replace the karyogamy
of gametes (e. g. a prolonged rest of the gametogonial
cell of Botrydium gives its brood-cells a power of independent
development instead of the tendency to unite as
gametes).
9. Those organisms that have attained the capability of
karyogamic rejuvenescence may, by prolonged fissile reproduction
without karyogamy, pass into a senile condition; marked
by reproductive incapacity. In these, therefoi'e, karyogamic
rejuvenescence has become essential to the preservation of the
race.
10. Rapidly repeated nuclear fissions, without sufficient
interval for nutrition and recovery, may lower the vital energy
or constitution of the cell, and accelerate this reproductive
incapacity; and this may be the physiological import of the
fissions that so frequently differentiate the gamete, and determine
its obligatory character.
11. The reproductive incapacity of most microgametes finds,
however, a sufficient explanation in the extreme reduction of
their cytoplasm.
12. The reproductive incapacity due to long or rapidly repeated
acts of fission uninterrupted by karyogamy is a matter