I.
PROBLEM
OF THE
ORGANIZER
3
To avoid misunderstanding, it may be convenient to clarify the use
of a few terms employed repeatedly in the present article. The term
organizer is used according to Spemann (1936) to denote a living
embryonic area or a piece of it endowed with the special faculties
suggested above, but not to denote a chemical or physical entity
responsible for the organizer action, for which expressions such as agent
or factor are reserved. The word inductor indicates any living or nonliving system which has an inducing effect on a competent reacting
system. Embryonic induction or simply induction is used whenever the
mode of differentiation of a developing system is obviously changed
under the influence of another developing system or of a factor coming
from the outside. The word is used regardless of whether the outcome
of differentiation of the reacting system is simple or complex, organized
or unorganized, and whether the underlying mechanism conforms with
that of normal induction or not. Words such as evocation, individuation,
and dorsalization are used according to the meanings defined by their
proposers.
II. Fine Structure of Embryonic Cells
as Revealed by Electron Micrographs
It is rather surprising that until quite recently many developmental
phenomena, particularly embryonic induction in vertebrate embryos,
have been discussed without essential knowledge of subcellular components of the cells which are participating in the phenomenon. This
was chiefly due to the fact that the embryonic cells of vertebrates is a
difficult subject for the methods of classical cytology. The introduction
of cytochemical methods helped to clarify many of the problems, but it
is undeniable that these methods have their limitations. Only with
electron microscopic study of thin sections has it become possible to
identify and characterize the essential morphological components of
these cells. In this section, subcellular components of the amphibian
embryonic cells which participate in the organizer action will be described,
and their possible role in induction and differentiation will be discussed.
According to the electron microscopic studies of Eakin and Lehmann
(1957), and Karasaki (1959a), the subcellular components recognized in
the cytoplasm of the amphibian embryonic cell, during the early phase
of development are as follows: Mitochondria, lipochondria (lipoid
droplets), micro-particles, cytoplasmic vesicles (endoplasmic reticulum),
pigment granules, Golgi complex, yolk platelets, and cell membrane.
A. Mitochondria
The mitochondrial population in the ectodermal cell, which has been
studied most carefully, reveals a progressive maturation in morphology
PROBLEM
OF THE
ORGANIZER
3
To avoid misunderstanding, it may be convenient to clarify the use
of a few terms employed repeatedly in the present article. The term
organizer is used according to Spemann (1936) to denote a living
embryonic area or a piece of it endowed with the special faculties
suggested above, but not to denote a chemical or physical entity
responsible for the organizer action, for which expressions such as agent
or factor are reserved. The word inductor indicates any living or nonliving system which has an inducing effect on a competent reacting
system. Embryonic induction or simply induction is used whenever the
mode of differentiation of a developing system is obviously changed
under the influence of another developing system or of a factor coming
from the outside. The word is used regardless of whether the outcome
of differentiation of the reacting system is simple or complex, organized
or unorganized, and whether the underlying mechanism conforms with
that of normal induction or not. Words such as evocation, individuation,
and dorsalization are used according to the meanings defined by their
proposers.
II. Fine Structure of Embryonic Cells
as Revealed by Electron Micrographs
It is rather surprising that until quite recently many developmental
phenomena, particularly embryonic induction in vertebrate embryos,
have been discussed without essential knowledge of subcellular components of the cells which are participating in the phenomenon. This
was chiefly due to the fact that the embryonic cells of vertebrates is a
difficult subject for the methods of classical cytology. The introduction
of cytochemical methods helped to clarify many of the problems, but it
is undeniable that these methods have their limitations. Only with
electron microscopic study of thin sections has it become possible to
identify and characterize the essential morphological components of
these cells. In this section, subcellular components of the amphibian
embryonic cells which participate in the organizer action will be described,
and their possible role in induction and differentiation will be discussed.
According to the electron microscopic studies of Eakin and Lehmann
(1957), and Karasaki (1959a), the subcellular components recognized in
the cytoplasm of the amphibian embryonic cell, during the early phase
of development are as follows: Mitochondria, lipochondria (lipoid
droplets), micro-particles, cytoplasmic vesicles (endoplasmic reticulum),
pigment granules, Golgi complex, yolk platelets, and cell membrane.
A. Mitochondria
The mitochondrial population in the ectodermal cell, which has been
studied most carefully, reveals a progressive maturation in morphology