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478
in such an assumption are appreciated. It has, however, been
possible to examine the number of somatic chromosomes in an
"extracted^ sylvestris which has bred true for one generation after
the back-cross. The number of chromosomes is apparently 2x—24
like the parent sylvestris. The explanation of this situation given
above would involve the considerable assumption that in some
cases the 12 sylvestris bivalent partners all pass to one pole in the
helerotypie anaphase. The difficulties of interpreting the recovery
of the Tabacum set of chromosomes is, obviously, greater. It is
hoped in the near future to present further cytological evidence
bearing on these matters.
Sacusowa (C. R. Soc. Sc. Varsovie, 1917) and later Woycicxi (Dis.
Biol. Arch. Soc. Sc. Varsaviensis, 1921) have published partially
complete accounts of the cytology of a Tabacum X sylvestris hybrid.
Apparently neither invesligalor realized that Ihe parents of this
hybrid differ in chromosome number and this fact, perhaps, explains
their lack of appreciation of the phenomena which they describe.
Wovcickri has figured many of the significant stages observed by
the present writer. In the helerotvpic metaphase he reports twenty-
four chromosomes of which twelve are large and twelve smaller
but for some reason speaks of them as paired ("douze paires de
chromosomes, etc"). SACHSowA lays emphasis upon what appear
to be abnormalities produced by fixation in early metaphase and
which are figured as a string of thirteen chromosomes. He appa-
rently considers this to be the haploid number and further con-
cludes that the Drosera scheme does not apply. He, however, figures
bivalent and univalent chromosomes behaving in a manner corres-
ponding to the stages illustrated and discussed by the present writer.
The studies reported upon in this paper were carried on in the
Botanical Institute of the Universily of Stockholm while the author
held a Fellowship under the American Scandinavian Foundation.
It is a pleasure to acknowledge my indebtedness to Professor O.
ROSENBERG and the members of the Botanical Institute for many
courlesies and much assistance and advice. Iam further indebted
to Professor R. R. GATES through whose courtesy I was enabled
lo grow a number of cultures in the Garden of the Royal Botanic
Sociely, Regents Park.
Botanical Institute University of Stockholm.
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