34
J. B . G U R D O N
in Sciara, where the chromosome set derived from the fly's father is
always eliminated in an atypical meiosis during spermatogenesis
(Smith-Stocking, 1936). These changes affect only whole chromosome
sets and are therefore unlikely to have a pronounced influence on the
control of gene activity.
Chromosome changes which appear to alter the balance of genes in a
nucleus regularly occur in the development of some organisms. In
Ascaris one pair of chromosomes is present in the zygote, but these
fragment into several pairs during the first few cleavage divisions of all
cells except the prospective germ cells (Boveri, 1910). Boveri also
showed by centrifugation experiments that the type of cytoplasm in
which a nucleus lies will determine whether its chromosomes undergo
elimination. In Cecidomyid flies the great majority of chromosomes are
eliminated between the fifth and seventh cleavage divisions of the
somatic nuclei. Only the nuclei at one pole of the egg, those destined to
participate in germ-cell formation, retain the full complement of
chromosomes (Geyer-Duszynska, 1959). Centrifugation experiments
have again shown that elimination is determined by the type of cytoplasm in which a nucleus lies. We know that nuclei which have eliminated chromosomes cannot support germ-cell formation, but there is
no reason to think that elimination plays any part in promoting somatic
cell differentiation. Several examples are known in which chromatin
bodies or other intranuclear inclusions are associated with germ cells
(review by Beermann, 1956). Since these bodies appear to be found only
in germ cells, their function is probably unconnected with the specialization of somatic cells.
I t seems clear that these examples of chromosome changes which
occur regularly in development are peculiar to the organisms in which
they have been reported and do not exemplify a general phenomenon
which is hard to detect in other organisms. These changes occur in only
a few cell types of a few organisms. Furthermore they are usually completed at one precise point in development and are not progressive like
cell differentiation appears to be. There is therefore no evidence against
the conclusion reached from nuclear transfer experiments that the somatic nuclei of an individual are genetically identical (except for occasional
spontaneous mutations), and that quantitative or qualitative changes
in the genetic material play no part in promoting cell differentiation.
B. Stable Activation or Repression of Genetic Material
The following discussion is devoted to stable changes in the expression
of genes which are themselves unchanged. Concrete examples of this
kind are hard to establish since it is difficult to be sure that a change in
gene expression does not involve a change in the gene itself and that the
J. B . G U R D O N
in Sciara, where the chromosome set derived from the fly's father is
always eliminated in an atypical meiosis during spermatogenesis
(Smith-Stocking, 1936). These changes affect only whole chromosome
sets and are therefore unlikely to have a pronounced influence on the
control of gene activity.
Chromosome changes which appear to alter the balance of genes in a
nucleus regularly occur in the development of some organisms. In
Ascaris one pair of chromosomes is present in the zygote, but these
fragment into several pairs during the first few cleavage divisions of all
cells except the prospective germ cells (Boveri, 1910). Boveri also
showed by centrifugation experiments that the type of cytoplasm in
which a nucleus lies will determine whether its chromosomes undergo
elimination. In Cecidomyid flies the great majority of chromosomes are
eliminated between the fifth and seventh cleavage divisions of the
somatic nuclei. Only the nuclei at one pole of the egg, those destined to
participate in germ-cell formation, retain the full complement of
chromosomes (Geyer-Duszynska, 1959). Centrifugation experiments
have again shown that elimination is determined by the type of cytoplasm in which a nucleus lies. We know that nuclei which have eliminated chromosomes cannot support germ-cell formation, but there is
no reason to think that elimination plays any part in promoting somatic
cell differentiation. Several examples are known in which chromatin
bodies or other intranuclear inclusions are associated with germ cells
(review by Beermann, 1956). Since these bodies appear to be found only
in germ cells, their function is probably unconnected with the specialization of somatic cells.
I t seems clear that these examples of chromosome changes which
occur regularly in development are peculiar to the organisms in which
they have been reported and do not exemplify a general phenomenon
which is hard to detect in other organisms. These changes occur in only
a few cell types of a few organisms. Furthermore they are usually completed at one precise point in development and are not progressive like
cell differentiation appears to be. There is therefore no evidence against
the conclusion reached from nuclear transfer experiments that the somatic nuclei of an individual are genetically identical (except for occasional
spontaneous mutations), and that quantitative or qualitative changes
in the genetic material play no part in promoting cell differentiation.
B. Stable Activation or Repression of Genetic Material
The following discussion is devoted to stable changes in the expression
of genes which are themselves unchanged. Concrete examples of this
kind are hard to establish since it is difficult to be sure that a change in
gene expression does not involve a change in the gene itself and that the