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

SOME PROBLEMS OF BJEPBODUCTION. 75
assume the behaviour of zygotes by developing as normal
embryos. 1
6. PARAGAMY comprises those cases where the fusion of
sister-nuclei replaces the advent of a male nucleus.
a. OOPARAGAMY occurs in the Metazoa in this wise: after
the first polar body is formed a second polar spindle is formed
in the egg, as if to form a second polar body; but the nucleus
corresponding with the second polar body moves back again
to fuse with the nascent oosphere nucleus. This has been
observed in Ascaris and Pterotrachea by Boveri, and
in Asteracanthion by 0. Hertwig. The fusion nucleus
thus formed has the same number of chromatic elements as the
normal zygote nucleus, double that of the gametogonium; and it
is essentially different from the nuclei produced by mere fission
during the whole cellular cycle since the last rejuvenescence.
b. APOCYTIAL PARAGAMY occurs when the fusion of the
nuclei of an apocytium wholly replaces the formation and
union of gametes. This occurs in Saprolegniese and
Derbesia.
IX. GENERAL CONCLUSIONS.
The following theses state concisely the results of our
inquiry:
1. Absolutely agamous forms exist in the group Monadinese;
in these REST is the only agent of rejuvenescence.
2. CHANGE OF THE MODE OF LIFE is a frequent mode of
rejuvenescence in apogamous and self-fertilising organisms.
3. In the higher Monadinese and the Myxomycetes a
plasmodium formation occurs, so that the cytoplasm is renewed
by PLASTOGAMY, and the nuclei wander from their original
cytoplasts.
1 The apogamic development of the plant of Pteris cretica from a group
of cells in the prothallus, instead of from a fertilised oosphere, comes very
close to this group of processes. I should note that apogamy is a purely
negative word, implying solely the excision or loss of a sexual process
from the life-history of an organism ; but this may be effected and compensated
in most divers ways, which might be classified if it were not
rather outside the scope of the present essay to do so.