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

SOME PROBLEMS OF REPRODUCTION. 45
r/ 3 s-~z~]-
J/«"8- Z
*-u 3 m
•{!
v
V
FIG. 6.—Schemaof formationof pronuclei in a conjugating Ciliate. /t=original
microuuoleu3 of one of the gametes. /i 3 s and /« 3 m = the stationary and
migratory (female and male) pronuclei respectively. v 3 m = migratory
pronucleus from the other conjugate, which lies the other side of the
dotted line. The pronuclei are supposed to unite in the square Z. The
dash above indicates rejection-nuclei.
Some Ciliates have habitually two micronuclei; in this case
both undergo the first two mitoses to form eight nuclei (ft 2 ),
and seven of these abort, leaving only one to undergo the final
mitosis; or, again, one of the two micronuclei undergoes the
first mitosis only before its brood abort.
The conjugation-nucleus undergoes at least two mitoses,
and of the four nuclei so formed in the simplest cases two
become mega- and two micro-nuclei, and at the first fusion of
the exconjugate one mega- and one micro-nucleus pass to each
daughter-individual. Though this is the easiest type to
understand it is not the commonest, but the processes, though
'of interest, are complex and too remote from our subject.
Some of the progeny of the zygote-nucleus in certain species
are eliminated as rejection-nuclei—a very significant fact.
The main peculiarities of the conjugation in the Ciliata
(apart from the formation of rejection-nuclei, to which we shall
return) are—(1) the formation of two fertile pronuclei; (2)
the separation of the gametes after karyogamic union of the
nuclei has taken place without any transference of cytoplasm.
After noting that here, at least, conjugation is an essentially
nuclear process, we proceed to infer from the coexistence of
conditions 1 and 2 that they are correlated phenomena. Let
us consider the process after the second mitosis and elimination
of the three rejection-nuclei.