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

SOME PROBLEMS OF REPRODUCTION. 55
rods, into which the chromatin of the nucleus resolves itself
in the early stages (prophases) of mitosis.
It appears to be the rule that, apart from gametes and their
antecedent cells, all the nuclei of a given species have the same
number of chromatomeres; each of these during mitosis splits
into two, lengthwise, and one half goes to each daughter-nucleus.
In most flowering plants the normal number of chromatomeres
in the vegetative cell is 16; at a certain stage, anterior
to the gametogonium proper, the chromatin wreath (which
had shown 16 rods at its formation) now segments in its prophases
into a smaller number than were present in the metaphases
and anaphases of the previous mitosis, which number
is perpetuated in the gametogonia and gametes. Thus in
Helleborus and most Liliacese the reduced number is 12,
two thirds the original; in the Liliaceous genus Allium it is
8. But in Convallaria (Lily-of-the-valley), also belonging to
the same order, there is no reduction, and in Muscari (Grape
Hyacinth) it is raised to 24. In Orchids, also, no reduction
has been observed.
The cell in which this reduction first takes place does not
appear fully determined in all cases; but we know this much,
that in the male (anther) it furnishes vegetative as well as
reproductive offspring. For it occurs, according to Guignard, in
the pollen mother-cell, which forms four pollen grains, each
to produce a vegetative and a gametogenic nucleus. All
these perpetuate the reduced number of chromatomeres by
normal mitosis.
In the female (ovule) this reduction is first shown by the
original nucleus of the embryo-sac. In some cases, at least
(Lilium Martagon), Guignard, whose recent account is
slightly different from Strasbiirger's and from his own previous
statements, has shown 1 that the normal number of chromatotution
des Noyaux cellulaires chez les V6g6taux," ' Comptes Rendus,' May 11,
1891. See also Boveri's " Zellen Studien : Verlialten der chromatischen Kernsubstanz
b. d, Bildung der Richtungskorper u. b. d. Befruchtung," in 'Jen.
Zeit.,' 1890.
. i " Nouvelles Recherches sur le Noyau cellulaire," in ' Ann. des Sei.
Nat. Bot.,' ser. 6, xx, p. 334, and ' Const, des Noyaus:'