I. PROBLEM OF THE
ORGANIZER
15
seem to survive. In a study of surviving cells during the phase of
recovery the following events appear to be of importance: (i) the
disappearance of the superficial layer of the yolk platelets; (ii) the
appearance of a large number of fine dense particles and small cytoplasmic vesicles within the cytoplasm (Fig. 4). Probably the particles
composing the external layer of the yolk platelets are set free by the
shock treatment and discharged into the cytoplasmic matrix. It should
be remembered that according to histochemical data these basophilic
particles appear to be acid polysaccharide and not ribonucleoprotein
(Section II.D). Consequently the observed sudden discharge of a large
amount of mucous substance from ammonia-treated cells can be
interpreted by assuming release of these mucopolysaccharide particles
from the cells. In fact, the electron micrographs prepared by Karasaki
of the mucous substance attached to the surface of ammonia-treated
cells indicate that the substance is composed of dense particles very
similar to those found in the external layer of yolk platelets (Fig. 4).
One might further speculate that the large number of small vesicles
which appear with fine particles may function in releasing the particles
through the cell membrane. Such a mechanism apparently fits in well
with the current view concerning the passage of macromolecules across
the cell membrane (Bennett, 1956). Appearance of abundant basophilic
granules in the previously hyaline cytoplasm after a shock treatment of
the ectoderm cells was noticed by Holtfreter (1946). Brächet (1946)
published a short note reporting separation of basophilic material,
which he assumed to be ribonucleoprotein, from yolk in the cytolyzing
ectoderm cells, and its disappearance in the course of cytolysis.
Probably what was observed in these cases was the separation of the
particles of the external layer of the yolk platelets.
Apparently in the intact ectoderm cells the yolk platelets are
protected from enzymatic actions by the presence of the external layer
which has been shown to be resistant to proteolytic attack (Section
II.D). If this is the case, its removal by the shock treatment may create
a new situation in the cell, in which utilization of the main body of the
yolk can be realized by the action of enzymes such as cathepsin or
phosphoprotein phosphatase, whose presence in amphibian embryos
has been demonstrated (Harris, 1946; Barth and Barth, 1954; Flickinger,
1956; Deuchar, 1958; D'Amelio and Ceas, 1957). It should be remembered that solation of the internal cytoplasm occurs after the shock
treatment, and that the event may be essential for neuralization. It is
not improbable that solation of the endoplasm is the cause of discharge
of the particles of the yolk external layer.
Some important results were obtained by applying the ammonia
treatment to the presumptive mesoderm. The medio-ventral sector of
ORGANIZER
15
seem to survive. In a study of surviving cells during the phase of
recovery the following events appear to be of importance: (i) the
disappearance of the superficial layer of the yolk platelets; (ii) the
appearance of a large number of fine dense particles and small cytoplasmic vesicles within the cytoplasm (Fig. 4). Probably the particles
composing the external layer of the yolk platelets are set free by the
shock treatment and discharged into the cytoplasmic matrix. It should
be remembered that according to histochemical data these basophilic
particles appear to be acid polysaccharide and not ribonucleoprotein
(Section II.D). Consequently the observed sudden discharge of a large
amount of mucous substance from ammonia-treated cells can be
interpreted by assuming release of these mucopolysaccharide particles
from the cells. In fact, the electron micrographs prepared by Karasaki
of the mucous substance attached to the surface of ammonia-treated
cells indicate that the substance is composed of dense particles very
similar to those found in the external layer of yolk platelets (Fig. 4).
One might further speculate that the large number of small vesicles
which appear with fine particles may function in releasing the particles
through the cell membrane. Such a mechanism apparently fits in well
with the current view concerning the passage of macromolecules across
the cell membrane (Bennett, 1956). Appearance of abundant basophilic
granules in the previously hyaline cytoplasm after a shock treatment of
the ectoderm cells was noticed by Holtfreter (1946). Brächet (1946)
published a short note reporting separation of basophilic material,
which he assumed to be ribonucleoprotein, from yolk in the cytolyzing
ectoderm cells, and its disappearance in the course of cytolysis.
Probably what was observed in these cases was the separation of the
particles of the external layer of the yolk platelets.
Apparently in the intact ectoderm cells the yolk platelets are
protected from enzymatic actions by the presence of the external layer
which has been shown to be resistant to proteolytic attack (Section
II.D). If this is the case, its removal by the shock treatment may create
a new situation in the cell, in which utilization of the main body of the
yolk can be realized by the action of enzymes such as cathepsin or
phosphoprotein phosphatase, whose presence in amphibian embryos
has been demonstrated (Harris, 1946; Barth and Barth, 1954; Flickinger,
1956; Deuchar, 1958; D'Amelio and Ceas, 1957). It should be remembered that solation of the internal cytoplasm occurs after the shock
treatment, and that the event may be essential for neuralization. It is
not improbable that solation of the endoplasm is the cause of discharge
of the particles of the yolk external layer.
Some important results were obtained by applying the ammonia
treatment to the presumptive mesoderm. The medio-ventral sector of