Full resolution (JPEG) - On this page / på denna sida - Sidor ...
<< prev. page << föreg. sida << >> nästa sida >> next page >>
Below is the raw OCR text
from the above scanned image.
Do you see an error? Proofread the page now!
Här nedan syns maskintolkade texten från faksimilbilden ovan.
Ser du något fel? Korrekturläs sidan nu!
This page has never been proofread. / Denna sida har aldrig korrekturlästs.
477
their division the single chromosomes appear to arrange themselves
in the equatorial region much more uniformally than in a cor-
responding stage in P. M. C.
The chalazal daughter nucleus is always the more active and
passes into the second division in advance of the micıopylar, as
shown in fig. 2 c. In this division there is, apparently, much less
tendency to irregularity than in a similar.stage in P. M. C. As
to the condition of the chromosomes remaining and dividing in
the spindle in anaphase of the second division of both P. M. C.
and E. M. C., the following provisional assumption is made. First,
that those which appear to have divided in the equalorial region
and whose products actually are approaching the opposite poles
represent undivided univalents which, in the first division, started
to divide near one or other pole but were included entire in a
daughter nucleus. Second, that the, usually much smaller, chro-
mosomes which are merely segmenting in the equatorial region, or
out of it, represent the halves of univalents which fully divided and
separated in the first division and that, in this case, their division
in homotypic anaphase is not completed or, at least, that the
products do not enter into the subsequent formation of major nuclei.
The fate of the chromosomes in the development of the embryo-
sacs is of great importance in these hybrids because a few of the
embryo-sacs are fertile and give viable seeds when crossed back
to the parents. The results of these back-crosses are fully described
elsewhere (Univ. Calif. Publ. Botany, vol. 11, no. 1) but may be
summarized in very general fashion as follows. When sylvestris,
for example, is crossed back to F, the progeny contains plants
equivalent to sylvestris and also many abnormal forms. With
Tabacum the back-cross gives Tabacum, ihe F, and abnormal
forms. The “extracted“ parents are fertile and breed true, the
remainder of the populations is largely sterile.
The cytological evidence presented above is interpreted as in-
dicating that the 12,,+12, chromosomes present represent 12 syl-
vestris + 12 Tabacum and 12 unpaired Tabacum chromosomes.
With this explanation in mind and in view of the genelic evidence
summarized above it is suggested that a few of the embryo-sacs
capable of fertilization are matured with the pure set of 12 syl-
vestris chromosomes, others with the pure set of 24 Tabacum chro-
mosomes and a larger number with variously contaminated parent
sets in the significant nuclei. The theoretical dilficulties involved
<< prev. page << föreg. sida << >> nästa sida >> next page >>
