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

SOME PROBLEMS OF REPRODUCTION. 47
indeed, the fact that cytoplastic division remains in abeyance
during the absence of a meganucleus in " working order " is an
additional argument for regarding this as a directive organ.
We may note that the pairing individuals are in many cases
much reduced below the normal size by a rapidly repeated
series of bipartitions.
We may then briefly state our view of the homologies of the
process of conjugation thus:—(1) The three mitoses of the
micronucleus point to a primitive formation of gametes in
broods of eight. (2) The fact that no cell division accompanies
these mitoses may be the physiological result of the inertness
of the now disorganised meganucleus. (3) The formation of two
fertile pronuclei in each gamete points to an ancestral stage, in
which a binary division of two mutually attached individuals
immediately preceded the conjugation of their offspring. (4) The
interchange of pronuclei and separation of the gametes are
probably modifications of the process indicated in (3), directed
to the preservation of the highly organised cytoplast. (5)
The pairing Ciliata before the completion of the preliminary
mitoses are not true gametes, but progametes.
b. Vorticellinea.
The group of VORTICELLINES, consisting of attached forms,
is exceptional in two respects. (1) The conjugating individuals,
instead of being similar, are unlike and unequal; the
larger individual being of the ordinary attached type, the
smaller free, and produced by repeated vertical fissions from
an ordinary individual. (2) Instead of the gametes separating
ultimately, they fuse, the larger absorbing the smaller into
itself; and of the four pronuclei which are formed as in other
Ciliates, only two unite to form a single conjugation-nucleus.
The preliminary mitoses and formation of rejection-nuclei
offer no exceptional character, save that in the male two
micronuclei are formed (indicating a further suppressed fission).
Two pronuclei are formed in either gamete, and the
migratory pronuclei, instead of passing one another, fuse
where they meet; while the stationary pronuclei both abort.