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

SOME PROBLEMS OF REPRODUCTION. 67
VIII. PROTOPLASMIC REJUVENESCENCE,
ITS NATURE AND MODES.
From the degeneration and loss of constitutional vigour
produced by the over-prolonged association of nucleus and
cytoplast, unchanged through a long chain of fissions, the
escape lies through a REJUVENESCENCE of the " firm," as we
may term them. And this is effected in various ways.
A. THE MODES OF REJUVENESCENCE.
1. REST from a given stimulus is sufficient to rouse again
the irritability of a nerve-centre when not unduly fatigued.
Even the operative weaver or working engineer, who from
constant habit is barely conscious of the unceasing din of the
machinery, would feel it afresh after a few weeks' absence.
And in the resting cell the nucleus has, moreover, the opportunity
of complete nutritive restoration. In the agamous
Monadinese, resting states, more or less prolonged and
accentuated, separate the stages of active growth and fission.
Here, too, as so often occurs in plants of higher organisation,
the more marked resting state usually precedes a recrudescence
of active cell division.
2. CHANGE OF THE MODE OF LIFE is another mode of bringing
about an harmonious readjustment of the relations of
nucleus and cytoplast. It may be accomplished by mere
POLYMORPHISM, or by HETERCECISM, the change of host, so
frequent in the life cycles of parasitic organisms. Marshall
Ward has drawn attention to this in the case of the higher
Fungi which are so frequently apogamous. 1
3. NUCLEAR MIGRATION, i. e. the transference of a nucleus
to a portion of cytoplasm with which it has not been asso-
1 " On the Sexuality of the Fungi," in ' Quart. Journ. Micr. Sci.,' 1884;
see pp. 59, 60 (of reprint) especially, where Ward compares the sojourn in a
new host " to a trip to the sea-side, where the weary and enfeebled organism
enjoys fresh diet and associations for a time, which in their turn pall and
prepare their recipients to renew old modes of life."