76 MARCUS M. HARTOG. 
4. Isogamy, plural or binary, is a step in advance of plas- 
modium 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 essen- 
tially different from the fission nuclei of the previous cell- 
cycle. 
8. Other modes of rejuvenescence may replace the karyo- 
gamy of gametes (e. g. a prolonged rest of the gametogonial 
cell of Botrydium gives its brood-cells a power of inde- 
pendent development instead of the tendency to unite as 
gametes). 
9. Those organisms that have attained the capability of 
karyogamic rejuvenescence may, by prolonged fissile reproduc- 
tion without karyogamy, pass into a senile condition marked 
by reproductive incapacity. In these, therefore, 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 tlie 
fissions that so frequently differentiate the gamete, and deter- 
mine 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 re- 
peated acts of fission uninterrupted by karyogamy is a matter 
