EARLY DEVELOPMENT OF THE SEA URCHIN
129
the blastula stage, thymidine was already incorporated in the zygote
nucleus.
B. The Mitochondria
Mitochondria have always attracted a great deal of attention on the
part of both cytologists and embryologists regarding the function they
may perform in the life-cycle of the cells. The investigations of Benda
(1903), soon followed by those of Meves (1908), Luna (1913a, b) and
others, indicated the mitochondria as a sort of precursor for a number of
cell structures and cell products. In fact, they were presumed to give
rise to, and indeed to become transformed into, the secretory granules
of the glandular cells, the myofibrils in the myoblasts, the neurofibrils
in the neuroblasts, etc. These ideas, however, were sooner or later
abandoned as the methods upon which the observations relied proved to
be fallacious or inadequate. The great merit of the methods of isolation
of the cell particulates has been to show, beyond any reasonable doubt,
that the mitochondria are the site of the respiratory enzymes and of the
energy-yielding systems. From this point of view, therefore, there is no
doubt that mitochondria play an important role in the course of
embryonic development, just as in any other cell in which there is a
great demand for energy. This, however, is what might be called a
general metabolic role. The question the embryologist is interested in is
whether there is any clear-cut, or even suggestive, evidence that
mitochondria might play a more specific, morphogenetic role (see also
Novikoff, 1961 ; Weber and Boell, 1962).
Changes in the shape and internal structure of mitochondria in the
course of development and in the different territories of the embryo have
recently been revealed by electron microscopic investigations. In the
amphibian embryo mitochondria not only have a different appearance
in the different ectodermal regions but they also appear to undergo
progressive changes in the course of development (Eakin and Lehmann,
1957; Karasaki, 1959; Weber and Boell, 1962). This seems to suggest a
process of mitochondrial maturation (Karasaki, 1959) and of mitochondrial differentiation (Eakin and Lehmann, 1957). Also, in the egg
of Tubifex (Weber, 1956) the shape and the intracellular arrangement of
mitochondria are different in the different prospective cells as early as in
the somatoblast stage. The observation that in the amphibian embryo
the specific activity of the cytochrome oxidase—as measured on the
isolated mitochondria—increases during the course of development
(Weber and Boell, 1955) may also be taken as an indication of a biochemical mitochondrial differentiation during development.
So far such evidence is not available for the sea urchin embryo.
Indeed it has been found that the specific activity of the cytochrome
129
the blastula stage, thymidine was already incorporated in the zygote
nucleus.
B. The Mitochondria
Mitochondria have always attracted a great deal of attention on the
part of both cytologists and embryologists regarding the function they
may perform in the life-cycle of the cells. The investigations of Benda
(1903), soon followed by those of Meves (1908), Luna (1913a, b) and
others, indicated the mitochondria as a sort of precursor for a number of
cell structures and cell products. In fact, they were presumed to give
rise to, and indeed to become transformed into, the secretory granules
of the glandular cells, the myofibrils in the myoblasts, the neurofibrils
in the neuroblasts, etc. These ideas, however, were sooner or later
abandoned as the methods upon which the observations relied proved to
be fallacious or inadequate. The great merit of the methods of isolation
of the cell particulates has been to show, beyond any reasonable doubt,
that the mitochondria are the site of the respiratory enzymes and of the
energy-yielding systems. From this point of view, therefore, there is no
doubt that mitochondria play an important role in the course of
embryonic development, just as in any other cell in which there is a
great demand for energy. This, however, is what might be called a
general metabolic role. The question the embryologist is interested in is
whether there is any clear-cut, or even suggestive, evidence that
mitochondria might play a more specific, morphogenetic role (see also
Novikoff, 1961 ; Weber and Boell, 1962).
Changes in the shape and internal structure of mitochondria in the
course of development and in the different territories of the embryo have
recently been revealed by electron microscopic investigations. In the
amphibian embryo mitochondria not only have a different appearance
in the different ectodermal regions but they also appear to undergo
progressive changes in the course of development (Eakin and Lehmann,
1957; Karasaki, 1959; Weber and Boell, 1962). This seems to suggest a
process of mitochondrial maturation (Karasaki, 1959) and of mitochondrial differentiation (Eakin and Lehmann, 1957). Also, in the egg
of Tubifex (Weber, 1956) the shape and the intracellular arrangement of
mitochondria are different in the different prospective cells as early as in
the somatoblast stage. The observation that in the amphibian embryo
the specific activity of the cytochrome oxidase—as measured on the
isolated mitochondria—increases during the course of development
(Weber and Boell, 1955) may also be taken as an indication of a biochemical mitochondrial differentiation during development.
So far such evidence is not available for the sea urchin embryo.
Indeed it has been found that the specific activity of the cytochrome