I.
PROBLEM
OF THE
ORGANIZER
11
F. Lipochondria
Lipochondria (lipoid droplets) appear as lobes with a wavy outline
varying in size from 0-5 to 0-2 μ. The wavy outline may be an artifact
caused by fixation. Both Eakin and Lehmann (1957) and Karasaki
(1959a) distinguish two types of lipochondria. In one of them (type A),
the interior of the lobe appears very dark, while in the other (type B),
it has a spongy appearance in low-power electron micrographs, which is
caused by fine particles arranged irregularly. In both types, a distinct
limiting membrane is observable. Lipochondria are present in large
numbers in all types of embryonic cells throughout the early stages of
development.
G. Cell Membrane
In the cells of amphibian embryos the external surface of the cytoplasm is bordered with a single membrane of ca. 80 Á in thickness. The
large intercellular spaces which are common in the cleavage stage
become more and more restricted, so that at the beginning of gastrulation an extended contact area is established between adjoining cells.
In electron micrographs of the contact area, the cell membranes of two
adjoining ectodermal or mesodermal cells run parallel to each other,
being separated by an interspace of 200 Á or more, which either appears
empty or contains small particles.
H. Nucleus
The nucleoplasm of resting nuclei has a definite tendency to increase
in density during early development. In the ectodermal cell of Triturus
the nucleolus is absent until the blástula stage, appears during gastrulation, and assumes the typical structure at the tail-bud stage (Karasaki,
1959b). The observation confirms the early report of Brächet (1952)
based on cytochemical data. The nuclear envelope consisting of a double
membrane with pores shows active form changes during neurulation
(Karasaki, 1959b). One type of change is the formation of a deep
indentation of the nuclear surface in which part of the cytoplasm is
engulfed. Another change consists in protrusion of the external nuclear
membrane into the cytoplasm, often with the nucleolus in the vicinity.
This picture has been observed in actively differentiating tissue cells
(chick mesenchymal cells, Watanabe, 1957; young lymphocytes,
Tanaka, 1957; cartilage cells in regeneration, Hay, 1958), and interpreted as an expression of active nucleo-cytoplasmic interaction. At
about the same stage direct continuation of the endoplasmic reticulum
with the outer nuclear membrane is sometimes witnessed. Of some
PROBLEM
OF THE
ORGANIZER
11
F. Lipochondria
Lipochondria (lipoid droplets) appear as lobes with a wavy outline
varying in size from 0-5 to 0-2 μ. The wavy outline may be an artifact
caused by fixation. Both Eakin and Lehmann (1957) and Karasaki
(1959a) distinguish two types of lipochondria. In one of them (type A),
the interior of the lobe appears very dark, while in the other (type B),
it has a spongy appearance in low-power electron micrographs, which is
caused by fine particles arranged irregularly. In both types, a distinct
limiting membrane is observable. Lipochondria are present in large
numbers in all types of embryonic cells throughout the early stages of
development.
G. Cell Membrane
In the cells of amphibian embryos the external surface of the cytoplasm is bordered with a single membrane of ca. 80 Á in thickness. The
large intercellular spaces which are common in the cleavage stage
become more and more restricted, so that at the beginning of gastrulation an extended contact area is established between adjoining cells.
In electron micrographs of the contact area, the cell membranes of two
adjoining ectodermal or mesodermal cells run parallel to each other,
being separated by an interspace of 200 Á or more, which either appears
empty or contains small particles.
H. Nucleus
The nucleoplasm of resting nuclei has a definite tendency to increase
in density during early development. In the ectodermal cell of Triturus
the nucleolus is absent until the blástula stage, appears during gastrulation, and assumes the typical structure at the tail-bud stage (Karasaki,
1959b). The observation confirms the early report of Brächet (1952)
based on cytochemical data. The nuclear envelope consisting of a double
membrane with pores shows active form changes during neurulation
(Karasaki, 1959b). One type of change is the formation of a deep
indentation of the nuclear surface in which part of the cytoplasm is
engulfed. Another change consists in protrusion of the external nuclear
membrane into the cytoplasm, often with the nucleolus in the vicinity.
This picture has been observed in actively differentiating tissue cells
(chick mesenchymal cells, Watanabe, 1957; young lymphocytes,
Tanaka, 1957; cartilage cells in regeneration, Hay, 1958), and interpreted as an expression of active nucleo-cytoplasmic interaction. At
about the same stage direct continuation of the endoplasmic reticulum
with the outer nuclear membrane is sometimes witnessed. Of some