I.
PROBLEM OF THE ORGANIZER
47
is connected with its morphogenetic activities. Such synthetic activity
of the organizer seems to be reflected in the specific antigenicity of the
dorsal lip of the blastopore detected by Vainio (1956).
The quantitative autoradiographic study of incorporation of
1 4
C0 2
into the gastrulae of axolotl reported recently by Tencer (1958) reveals
another aspect of isotope uptake. The major portion of radioactivity is
present in the form of RNA which is distributed preferentially in the
ectodermal area, and the minor portion of the radioactivity resides in
proteins which are rather evenly distributed in all areas. In the early
gastrula the nuclei of all areas show the same level of activity, while in
the late gastrula the nuclei show an activity gradient conforming to the
usual animal-vegetal and dorso-ventral gradient. On the other hand in
the cytoplasm where the activity is always lower than that of the
nuclei, the usual gradient is established in the two stages. In the overall
pattern the radioactivity indicates the typical pattern of animal-vegetal
and dorso-ventral gradients.
An interesting attempt is being made by Flickinger and his co-workers
to correlate yolk utilization with the inductive ability of the organizer.
According to Rounds and Flickinger (1958), yolk breaks down in the
mesoderm of Rana embryos during its invagination, and RNA which is
originally present in the yolk fraction of the mesoderm is released and
taken up by the cytoplasmic granules of the mesoderm. That the yolk
platelets begin to show signs of utilization in the Rana embryo at this
stage has been confirmed also in electron micrographs of Karasaki
(unpublished data). However, it should be remembered that our
histochemical data (Section II, D) do not support the idea that RNA is
present in any appreciable amount in the yolk platelets. The data of
Rounds and Flickinger, which support this idea are based on a method
of fractionation in which RNA from mitochondria, pigment granules,
nuclei and a part of the microsome fraction might be included in the
yolk fraction. Thus the problem needs re-examination. In passing,
Rounds and Flickinger did not find the increase in the RNA content in
the neural ectoderm during its determination, when DNA was used as
the basis. However, the data do not necessarily contradict the idea of
transport of RNA from the mesoderm to the ectoderm, because the rate
of increase of DNA may be larger in the neural area than in the mesoderm. That the RNA content based on total nitrogen increases in the
presumptive neural ectoderm during gastrulation was shown by
K. Takata (1953) in the Triturus embryo.
IX. Concluding Remarks
From the viewpoint that cellular differentiation depends essentially
upon production of a set of specific proteins, embryonic induction may
PROBLEM OF THE ORGANIZER
47
is connected with its morphogenetic activities. Such synthetic activity
of the organizer seems to be reflected in the specific antigenicity of the
dorsal lip of the blastopore detected by Vainio (1956).
The quantitative autoradiographic study of incorporation of
1 4
C0 2
into the gastrulae of axolotl reported recently by Tencer (1958) reveals
another aspect of isotope uptake. The major portion of radioactivity is
present in the form of RNA which is distributed preferentially in the
ectodermal area, and the minor portion of the radioactivity resides in
proteins which are rather evenly distributed in all areas. In the early
gastrula the nuclei of all areas show the same level of activity, while in
the late gastrula the nuclei show an activity gradient conforming to the
usual animal-vegetal and dorso-ventral gradient. On the other hand in
the cytoplasm where the activity is always lower than that of the
nuclei, the usual gradient is established in the two stages. In the overall
pattern the radioactivity indicates the typical pattern of animal-vegetal
and dorso-ventral gradients.
An interesting attempt is being made by Flickinger and his co-workers
to correlate yolk utilization with the inductive ability of the organizer.
According to Rounds and Flickinger (1958), yolk breaks down in the
mesoderm of Rana embryos during its invagination, and RNA which is
originally present in the yolk fraction of the mesoderm is released and
taken up by the cytoplasmic granules of the mesoderm. That the yolk
platelets begin to show signs of utilization in the Rana embryo at this
stage has been confirmed also in electron micrographs of Karasaki
(unpublished data). However, it should be remembered that our
histochemical data (Section II, D) do not support the idea that RNA is
present in any appreciable amount in the yolk platelets. The data of
Rounds and Flickinger, which support this idea are based on a method
of fractionation in which RNA from mitochondria, pigment granules,
nuclei and a part of the microsome fraction might be included in the
yolk fraction. Thus the problem needs re-examination. In passing,
Rounds and Flickinger did not find the increase in the RNA content in
the neural ectoderm during its determination, when DNA was used as
the basis. However, the data do not necessarily contradict the idea of
transport of RNA from the mesoderm to the ectoderm, because the rate
of increase of DNA may be larger in the neural area than in the mesoderm. That the RNA content based on total nitrogen increases in the
presumptive neural ectoderm during gastrulation was shown by
K. Takata (1953) in the Triturus embryo.
IX. Concluding Remarks
From the viewpoint that cellular differentiation depends essentially
upon production of a set of specific proteins, embryonic induction may