18
J. B. GURDON
the transplant-embryos derived from endoderm nuclei are consistently
different from those derived from ectoderm or mesoderm nuclei, it is
difficult to understand how such a difference could be caused by technical factors during transplantation. This would then indicate that these
nuclear changes had arisen before transplantation in the course of
normal development. Briggs and King (1957, 1960) have described the
abnormalities characteristic of transplant-embryos derived from Rana
endoderm nuclei. These include the abnormal differentiation and early
degeneration of ectodermal and mesodermal derivatives compared to
the endodermal derivatives (Fig. 5, M-P). Owing to the technical
difficulty of transplanting the small ectoderm and mesoderm nuclei, we
do not know what kind of abnormalities these nuclei would yield, and
therefore whether they would differ from the endoderm syndrome
outlined above. Transplant-embryos derived from Xenopus endoderm
nuclei have not shown any obvious pattern of abnormalities (Gurdon,
1960b). In view of these points, it is premature to claim that the stable
nuclear changes, whose existence has been clearly demonstrated, are
cell-type specific and that the time of their origin before or after transplantation has been established.
Of particular importance in connection with the significance of these
stable nuclear changes are some experiments with the nuclei of differentiated cells. McKinnell (1962) has obtained swimming tadpoles from the
injection of several adenocarcinoma cell nuclei into enucleated eggs of
Rana pipiens. Though not entirely normal, these tadpoles had welldeveloped skin, brain, notochord, muscle, and gut, and had diploid
nuclei. There is no direct proof that the nuclei of the tadpoles were
derived from one of the injected nuclei, though this is probably the
case, since enucleation of the recipient eggs is reported to be nearly
100% reliable. In Xenopus, the nuclei of normal intestinal epithelium
cells have been transplanted successfully. These cells are fully differentiated as is shown by their possession of a striated border. Entirely
normal tadpoles, some of which are now adult frogs, have been obtained
from the direct and serial transplantation of the nuclei from these cells
(Gurdon, 1962c). Furthermore these tadpoles and frogs contained a
mutation which proved that their nuclei were derived from the transplanted donor nuclei. After including the results of serial transfers and
excluding cases of deficient and abortive cleavage as having been caused
by the technical difficulties mentioned above, altogether 70% of the
transplanted intestine nuclei gave tadpoles capable of muscular response. This means that at least 70% of the nuclei which had already
promoted the formation of specialized intestine cells had not undergone
any stable change or repression of those genes concerned in the differentiation of muscle and nerve cells. In spite of these results, much fewer
J. B. GURDON
the transplant-embryos derived from endoderm nuclei are consistently
different from those derived from ectoderm or mesoderm nuclei, it is
difficult to understand how such a difference could be caused by technical factors during transplantation. This would then indicate that these
nuclear changes had arisen before transplantation in the course of
normal development. Briggs and King (1957, 1960) have described the
abnormalities characteristic of transplant-embryos derived from Rana
endoderm nuclei. These include the abnormal differentiation and early
degeneration of ectodermal and mesodermal derivatives compared to
the endodermal derivatives (Fig. 5, M-P). Owing to the technical
difficulty of transplanting the small ectoderm and mesoderm nuclei, we
do not know what kind of abnormalities these nuclei would yield, and
therefore whether they would differ from the endoderm syndrome
outlined above. Transplant-embryos derived from Xenopus endoderm
nuclei have not shown any obvious pattern of abnormalities (Gurdon,
1960b). In view of these points, it is premature to claim that the stable
nuclear changes, whose existence has been clearly demonstrated, are
cell-type specific and that the time of their origin before or after transplantation has been established.
Of particular importance in connection with the significance of these
stable nuclear changes are some experiments with the nuclei of differentiated cells. McKinnell (1962) has obtained swimming tadpoles from the
injection of several adenocarcinoma cell nuclei into enucleated eggs of
Rana pipiens. Though not entirely normal, these tadpoles had welldeveloped skin, brain, notochord, muscle, and gut, and had diploid
nuclei. There is no direct proof that the nuclei of the tadpoles were
derived from one of the injected nuclei, though this is probably the
case, since enucleation of the recipient eggs is reported to be nearly
100% reliable. In Xenopus, the nuclei of normal intestinal epithelium
cells have been transplanted successfully. These cells are fully differentiated as is shown by their possession of a striated border. Entirely
normal tadpoles, some of which are now adult frogs, have been obtained
from the direct and serial transplantation of the nuclei from these cells
(Gurdon, 1962c). Furthermore these tadpoles and frogs contained a
mutation which proved that their nuclei were derived from the transplanted donor nuclei. After including the results of serial transfers and
excluding cases of deficient and abortive cleavage as having been caused
by the technical difficulties mentioned above, altogether 70% of the
transplanted intestine nuclei gave tadpoles capable of muscular response. This means that at least 70% of the nuclei which had already
promoted the formation of specialized intestine cells had not undergone
any stable change or repression of those genes concerned in the differentiation of muscle and nerve cells. In spite of these results, much fewer