I.
PEOBLEM
OF THE ORGANIZER
7
B. Ribonucleoprotein Particles and Cytoplasmic Vesicles
According to Karasaki (1959a and unpublished data) dense particles
having an average diameter of ca. 180 Á are scattered freely or in
clusters of several particles in the cytoplasm of early embryonic cells of
Triturus pyrrhogaster, Bufo vulgaris (Fig. 3), and Rana japónica. At the
tail-bud stage, although most of the particles are still found free or in
small clusters, some of the fine particles (micro-particles) are found
attached to the surface of cytoplasmic vesicles which have increased in
number at this stage. From the high RNA content of the ultracentrifugally prepared fraction of the fine particles (36% RNA) they may be
identified as ribonucleoprotein particles or Palade granules. There
apparently occurs an increase in the frequency of the fine particles in the
neural cell-line during the period between the gastrula and the tail-bud
stages. If we assume that the bulk of ribonucleoprotein of embryonic
cells is in the form of fine particles such an increase is to be expected
from the increase in the RNA content of the total embryo, as demonstrated by Steinert (1951) and of the neural area, as suggested with
histochemical methods by Brächet (1947a, b, 1957) or with chemical
analysis by Takata (1953). Also in embryonic cells of the early chick
embryo similar fine particles are shown in the electron micrographs of
Bellairs (1958, 1959).
Coming back to amphibian cells at the early gastrula stage, the
membranous structure of the endoplasmic reticulum system is represented only by small spherical or ovoidal vesicles, devoid of the fine
particles (smooth-surfaced). At this stage, these cytoplasmic vesicles are
very infrequent in Triturus ectoderm cells while somewhat more
frequent in Rana and Bufo ectoderm cells (Karasaki, 1959a and
unpublished data). An increase in the frequency and size of these
vesicles with the progress of development is clearly indicated. At the
tail-bud stage, many of the cytoplasmic vesicles are furnished with fine
particles attached on the external surface (rough-surfaced). In this case
also, membranes assume the form of a sphere, ellipsoid, or short tubule,
but never that of packed lamellae, although in the larval tissue of
Amphibia one often finds the condition of closely packed double
lamellae. The rough-surfaced cytoplasmic vesicles can be observed in
the epidermal cells as well as in the neural cells. According, to Bellairs
(1958, 1959) the rough-surfaced endoplasmic reticulum in the chick
embryo is present already at the primitive streak stage, though in small
amount. She made a quantitative estimation of the frequency of the
rough-surfaced endoplasmic reticulum per unit area of the section in
the neural cell-line, and found a strong increase during the period from the
long primitive streak stage to 10 days of incubation, in which the whole
PEOBLEM
OF THE ORGANIZER
7
B. Ribonucleoprotein Particles and Cytoplasmic Vesicles
According to Karasaki (1959a and unpublished data) dense particles
having an average diameter of ca. 180 Á are scattered freely or in
clusters of several particles in the cytoplasm of early embryonic cells of
Triturus pyrrhogaster, Bufo vulgaris (Fig. 3), and Rana japónica. At the
tail-bud stage, although most of the particles are still found free or in
small clusters, some of the fine particles (micro-particles) are found
attached to the surface of cytoplasmic vesicles which have increased in
number at this stage. From the high RNA content of the ultracentrifugally prepared fraction of the fine particles (36% RNA) they may be
identified as ribonucleoprotein particles or Palade granules. There
apparently occurs an increase in the frequency of the fine particles in the
neural cell-line during the period between the gastrula and the tail-bud
stages. If we assume that the bulk of ribonucleoprotein of embryonic
cells is in the form of fine particles such an increase is to be expected
from the increase in the RNA content of the total embryo, as demonstrated by Steinert (1951) and of the neural area, as suggested with
histochemical methods by Brächet (1947a, b, 1957) or with chemical
analysis by Takata (1953). Also in embryonic cells of the early chick
embryo similar fine particles are shown in the electron micrographs of
Bellairs (1958, 1959).
Coming back to amphibian cells at the early gastrula stage, the
membranous structure of the endoplasmic reticulum system is represented only by small spherical or ovoidal vesicles, devoid of the fine
particles (smooth-surfaced). At this stage, these cytoplasmic vesicles are
very infrequent in Triturus ectoderm cells while somewhat more
frequent in Rana and Bufo ectoderm cells (Karasaki, 1959a and
unpublished data). An increase in the frequency and size of these
vesicles with the progress of development is clearly indicated. At the
tail-bud stage, many of the cytoplasmic vesicles are furnished with fine
particles attached on the external surface (rough-surfaced). In this case
also, membranes assume the form of a sphere, ellipsoid, or short tubule,
but never that of packed lamellae, although in the larval tissue of
Amphibia one often finds the condition of closely packed double
lamellae. The rough-surfaced cytoplasmic vesicles can be observed in
the epidermal cells as well as in the neural cells. According, to Bellairs
(1958, 1959) the rough-surfaced endoplasmic reticulum in the chick
embryo is present already at the primitive streak stage, though in small
amount. She made a quantitative estimation of the frequency of the
rough-surfaced endoplasmic reticulum per unit area of the section in
the neural cell-line, and found a strong increase during the period from the
long primitive streak stage to 10 days of incubation, in which the whole