302 Strasburger , — The Periodic Reduction of 
This filament breaks up into a given number of segments 
simultaneously, not by successive divisions into two : it is 
on this account that such numbers as twelve are frequently 
found, numbers which cannot be the result of repeated 
division into two. The fact that, as a rule, the same number 
of chromosomes occurs in successive generations of nuclei, 
suggests the view that though the chromosomes may lose 
their morphological individuality in the resting nucleus, they 
do not lose their physiological individuality. The observa- 
tion of such a series of stages of nuclear division as can 
be obtained by the laying open of embryo-sacs in which 
the development of the endosperm is commencing, makes 
it difficult to resist the impression that it is always the same 
chromosomes which make their appearance over and over 
again in the repeated divisions. In the prophase the chromo- 
somes are seen to appear in precisely the same positions as 
they occupied in the preceding anaphase : and if the picture 
of the anaphase were proportionately enlarged it would 
exactly correspond to that of the succeeding prophase. In 
one word, it must be assumed that the individuality of the 
chromosomes persists in the resting nucleus, and determines 
the breaking up of the nuclear filament into the corresponding 
number of chromosomes in the succeeding prophase. Any 
change in the number of the chromosomes must be preceded 
by an alternation, whether increase or diminution, in the 
number of the chromosomatic individualities. The reduction 
of the number of the chromosomes by half, at the initiation 
of the sexual generation, is due to the fusion into one of two 
chromosomatic individuals, under the influence of causes 
which, for the present, can only be assumed on phylogenetic 
grounds. These fusions of chromosomes, at the initiation of 
the sexual generation, can apparently only take place under 
certain conditions. They are affected by abnormal internal 
changes : for instance, the embryonic substance constituting 
the growing-points of shoots affected with bud-variation often 
remains sterile ; and hybridization frequently induces similar 
consequences. 
