24
J . B. GURDON
when re-nucleated with discoides nuclei. I t would be very interesting to
know whether these changes in nucleus and cytoplasm are stable or
reversible and hence whether they reflect more than an initial response
to a foreign nucleus or cytoplasm.
4. Injection of Adult Tissue Substances into Eggs
Markert and Ursprung (1963) have carried out some experiments to
test the hypothesis that proteins associated with chromosomes may
give a stable repression of genes and so control their activity in development. They obtained various nuclear and cytoplasmic fractions
from the liver of adult Rana pipiens and injected small volumes of these
fractions into eggs after fertilization but before the first cleavage. They
found that the injection of all fractions as well as of saline solution
prevented some of the injected eggs from developing normally. However the injection of nuclear globulins or of cytoplasmic globulins +
albumins resulted in arrest at the blastula stage in nearly all eggs.
Recently Kimmel (1963) has shown that this effect is to some extent
species specific, since fractions of adult organs of other animals or of
frog gametes have a much weaker effect. By means of serial nuclear
transfers, Markert and Ursprung have shown that these liver fractions
exert their effect as a result of a stable nuclear change. An egg was
injected with a liver fraction and allowed to reach the blastula stage.
Before its cells had started dying, its nuclei were injected into enucleated
unfertilized eggs which had not received any liver fraction. After seven
serial-transfer generations, or 100 nuclear divisions, all the serial transfer embryos continued to be arrested at the blastula stage, while
10-20% of the control transfers from saline injected eggs reached the
tail-bud stage. The chromosome sets of the serial-transfer embryos very
frequently contained ring chromosomes as also did some of the salineinjected controls.
Markert and Ursprung have suggested that these results may reveal
a class of chromosomal proteins whose function is to regulate gene
activity by repression. If this is so, it is not quite clear why liver cell
proteins should repress enough genes to arrest development at the
blastula stage. As Ursprung and Markert (1963) have recently stated,
the chromosomal abnormalities are sufficient to account for the early
developmental arrest if it is assumed, as is quite reasonable, that the
extracted proteins have some harmful effect on chromosomes during
cleavage. This most recent opinion seems to account entirely satisfactorily for their results without the need to assume that the extracted
proteins have any regulatory function.
J . B. GURDON
when re-nucleated with discoides nuclei. I t would be very interesting to
know whether these changes in nucleus and cytoplasm are stable or
reversible and hence whether they reflect more than an initial response
to a foreign nucleus or cytoplasm.
4. Injection of Adult Tissue Substances into Eggs
Markert and Ursprung (1963) have carried out some experiments to
test the hypothesis that proteins associated with chromosomes may
give a stable repression of genes and so control their activity in development. They obtained various nuclear and cytoplasmic fractions
from the liver of adult Rana pipiens and injected small volumes of these
fractions into eggs after fertilization but before the first cleavage. They
found that the injection of all fractions as well as of saline solution
prevented some of the injected eggs from developing normally. However the injection of nuclear globulins or of cytoplasmic globulins +
albumins resulted in arrest at the blastula stage in nearly all eggs.
Recently Kimmel (1963) has shown that this effect is to some extent
species specific, since fractions of adult organs of other animals or of
frog gametes have a much weaker effect. By means of serial nuclear
transfers, Markert and Ursprung have shown that these liver fractions
exert their effect as a result of a stable nuclear change. An egg was
injected with a liver fraction and allowed to reach the blastula stage.
Before its cells had started dying, its nuclei were injected into enucleated
unfertilized eggs which had not received any liver fraction. After seven
serial-transfer generations, or 100 nuclear divisions, all the serial transfer embryos continued to be arrested at the blastula stage, while
10-20% of the control transfers from saline injected eggs reached the
tail-bud stage. The chromosome sets of the serial-transfer embryos very
frequently contained ring chromosomes as also did some of the salineinjected controls.
Markert and Ursprung have suggested that these results may reveal
a class of chromosomal proteins whose function is to regulate gene
activity by repression. If this is so, it is not quite clear why liver cell
proteins should repress enough genes to arrest development at the
blastula stage. As Ursprung and Markert (1963) have recently stated,
the chromosomal abnormalities are sufficient to account for the early
developmental arrest if it is assumed, as is quite reasonable, that the
extracted proteins have some harmful effect on chromosomes during
cleavage. This most recent opinion seems to account entirely satisfactorily for their results without the need to assume that the extracted
proteins have any regulatory function.