SOME PROBLEMS OF REPRODUCTION. | 67 
VIII. Protortasmic REJUVENESCENCE, 
irs Nature AND Mopss. 
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 ResuvenEscence of the “ firm,” as we 
may term them. And this is effected in various ways. 
A. Tur Mopes or 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 oppor- 
tunity of complete nutritive restoration. In the agamous 
Monadinee, 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 recrudes- 
cence of active cell division. 
2. CHANGE OF THE MODE OF LIFE is another mode of bring- 
ing about an harmonious readjustment of the relations of 
nucleus and cytoplast. It may be accomplished by mere 
POLYMORPHISM, or by HETER@CcISM, 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 
8. NUCLEAR MIGRATION, i.e. the transference of a nucleus 
to a portion of cytoplasm with which it has not been asso- 
1 “Qn the Sexuality of the Fungi,” in ‘ Quart. Journ. Mier. 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.” 
