42
TUNEO
YAMADA
that at least in the mouse embryo the mediating agent can pass through
pores of 0-1 μ. However, according to De Vicentiis (1954) lens induction
in Discoglossus pictus is prevented by placing a filter having large pores
(2-4 μ and 3-4 μ).
It should now be asked whether the alleged transmission of a particulate component of the organizer cell to the reacting ectoderm cell
has any support from electron microscopic data. Eakin and Lehmann
(1957) observed cytoplasmic bridges between adjacent cells of the early
gastrula of Xenopus laevis fixed with Lehmann's fixative, where a
direct cytoplasmic continuity without membranous barriers appears to
be established. However, they are careful to point out that the observation was made at a low magnification and should be checked by a
high magnification micrograph, and that material preserved with other
fixatives should also be studied for comparison. Further, they state,
without giving data, that 'evidence of breaches' was found between the
boundary of the presumptive neural ectoderm and underlying chordamesoderm, and suggest the possibility of a free transfer of macromolecular complexes of the inductor into the cells of the competent
ectoderm. Efforts are being made in our laboratory by Karasaki
(unpublished work) to obtain electron micrographs showing the condition of the cell contact in the boundary between the presumptive
neural ectoderm and the archenteric roof. In the area of close cell
contact in the boundary, an interspace of ca. 200 Á or more is present
between two cell membranes measuring ca. 80 Á each. A very irregular
course of the boundary is not uncommon. Often there are interruptions
of cell membranes and interspaces, where the cytoplasm of both cells
appears to be continuous (Fig. 15). Because of the exceptional fragility
of membranous structures of embryonic cells, it is very difficult to
decide whether the interruptions represent the condition during life or
are artifacts. Judging from the appearance of the cytoplasmic area
in the direct vicinity of the interruptions one gets the impression that
some of the interruptions are artifacts and others real. A large number
of fine particles which could be identified either as ribonucleoprotein
particles or as polysaccharide particles are present close to the cell
membranes within the cell. A small number of them may even be
observed within the interspace. Further, small cytoplasmic vesicles
often occur in the vicinity of the cell membranes or are fused with the
latter. This picture reminds us of the small vesicles observed in endothelial cells (Palade, 1953), Schwann cells and satellite cells of sympathetic neurones (De Robertis and Bennett, 1954), which are
interpreted in connection with pinocytosis, and assumed to function in
transportation of fluid across the cell membrane. Such a phenomenon
called membrane vesiculation (Bennett, 1956) may also occur in the
TUNEO
YAMADA
that at least in the mouse embryo the mediating agent can pass through
pores of 0-1 μ. However, according to De Vicentiis (1954) lens induction
in Discoglossus pictus is prevented by placing a filter having large pores
(2-4 μ and 3-4 μ).
It should now be asked whether the alleged transmission of a particulate component of the organizer cell to the reacting ectoderm cell
has any support from electron microscopic data. Eakin and Lehmann
(1957) observed cytoplasmic bridges between adjacent cells of the early
gastrula of Xenopus laevis fixed with Lehmann's fixative, where a
direct cytoplasmic continuity without membranous barriers appears to
be established. However, they are careful to point out that the observation was made at a low magnification and should be checked by a
high magnification micrograph, and that material preserved with other
fixatives should also be studied for comparison. Further, they state,
without giving data, that 'evidence of breaches' was found between the
boundary of the presumptive neural ectoderm and underlying chordamesoderm, and suggest the possibility of a free transfer of macromolecular complexes of the inductor into the cells of the competent
ectoderm. Efforts are being made in our laboratory by Karasaki
(unpublished work) to obtain electron micrographs showing the condition of the cell contact in the boundary between the presumptive
neural ectoderm and the archenteric roof. In the area of close cell
contact in the boundary, an interspace of ca. 200 Á or more is present
between two cell membranes measuring ca. 80 Á each. A very irregular
course of the boundary is not uncommon. Often there are interruptions
of cell membranes and interspaces, where the cytoplasm of both cells
appears to be continuous (Fig. 15). Because of the exceptional fragility
of membranous structures of embryonic cells, it is very difficult to
decide whether the interruptions represent the condition during life or
are artifacts. Judging from the appearance of the cytoplasmic area
in the direct vicinity of the interruptions one gets the impression that
some of the interruptions are artifacts and others real. A large number
of fine particles which could be identified either as ribonucleoprotein
particles or as polysaccharide particles are present close to the cell
membranes within the cell. A small number of them may even be
observed within the interspace. Further, small cytoplasmic vesicles
often occur in the vicinity of the cell membranes or are fused with the
latter. This picture reminds us of the small vesicles observed in endothelial cells (Palade, 1953), Schwann cells and satellite cells of sympathetic neurones (De Robertis and Bennett, 1954), which are
interpreted in connection with pinocytosis, and assumed to function in
transportation of fluid across the cell membrane. Such a phenomenon
called membrane vesiculation (Bennett, 1956) may also occur in the