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

SOME EBOBLEMS.Of BEPRODUCTION. 59
entiation, so that the indefinite reproductive capabilities of the
cells of Mosses and BegoniaSj Coelenterates and Flat-worms
are usually cited as exceptions to the rule; though the indefinite,
if not unlimited, capacity for fissile reproduction
must have been primitively inherent in every cell. The limitation
of this power in most Metazoa occurs at a very early
stage if the brilliant results of Chabry 1 prove to have a general
application^ for he has shown that in Tunicates the destruction
of a single blastomere may determine the absence of the
organ it should produce.
C. The Adaptation of Gametes to Different Fates.
Our next subject for parallel is the arrest or degradation of
certain gametes of a brood, or, if we please, the favouring of
single gametes of a brood at the expense of others. This is a
common phenomenon of the struggle for existence between
members of a colony, whether cells, organs, zooids, or even
individuals, especially those destined for reproduction. Thus,
in the ascus of the Truffle four to six spores are formed endogenously,
but usually only one matures. In the Heteror
sporous Filicinese sixty-four megaspores are formed (in
sixteen tetrads) in each megasporange; but only one matures,
the other sixty-three undergoing complete disorganisation. In
Eleocharis and other Sedges the pollen mother-cell undergoes
two unequal divisions, just like the Metazoan ovum, and
produces a single fertile pollen grain and three abortive ones.
The ovarian ovum of Hydra attains its full size by devouring
all the others in the ovary, and in many Arthropods the fertile
ova develop at the expense of others. In Ascaris megalocephala
some of the ova and spermatogonia are sacrificed and
abort in the germ-tubes (0. Hertwig). In Vertebrates also
many of the primitive ova are degraded to mere food material.
As for Organs and individuals, in Gymnosperms several
archegonia may be formed and fertilised in each embryo-sac;
1
" Embryologie Normale et T6ratologique des Asoidiens Simples," in
'Robin's Jour». del'Anat./1887, p. 167.