132
ALBERTO MONROY AND RACHELE MAGGIO
confine ourselves to mentioning very briefly some of the most pertinent
data. The most extensive studies on this subject have been made by
Reverberi and his collaborators on the ascidians (summarized in
Reverberi, 1961). Immediately after fertilization mitochondria accumulate at the vegetal pole of the egg, then in the yellow crescent and
finally in the primitive muscular cells and in the musculature of the
swimming larva (Reverberi, 1957). By the use of different inhibitors
Reverberi and his collaborators were then able to show that the
inhibition of mitochondrial enzymes results in alterations in the
territory where the mitochondria are accumulated. Although the interpretation of these observations is difficult, on the basis of available
evidence they certainly suggest that the differential accumulation of
mitochondria in the various territories of the embryo may play an
important morphogenetic role. Similar results have been obtained by a
number of other investigators on other eggs. The unequal distribution
of cytochrome oxidase among the different blastomeres of the Ascidian
embryo has been confirmed by Berg (1956).
The question of the gradient distribution of mitochondria in the sea
urchin embryo and its significance is discussed by Lallier (1964). In
this context it will only be mentioned that, according to Gustaf son and
Lenicque (1952, 1955), until the late blastula stage the mitochondria are
uniformly distributed throughout the embryo but at the mesenchyme
blastula stage an animal-vegetal gradient appears which becomes more
and more evident as development proceeds. However, the technique
used by Gustafson and Lenicque for counting mitochondria has been
questioned by Shaver (1956, 1957) who could not confirm their results.
More recently the problem of the gradient distribution of mitochondria
has been submitted to a careful electron microscopic analysis by Berg
et al. (1962) who have also been unable to obtain evidence of the
existence of such a gradient in the sea urchin blastula. On the other hand
they could detect a certain accumulation of yolk platelets in the vegetal
region of the embryo and in the cells of the primary mesenchyme. Some
of the observations of Gustafson and Lenicque were confirmed by
Shaver (1956,1957), e.g., the accumulation of mitochondria close to the
ciliated border and an increase of the mitochondria in the cells of the gut
and in the cells of the developing arms of the pluteus. Whether such
accumulations have a morphogenetic significance is doubtful.
The present situation is therefore quite perplexing. On the one hand
we find eggs in which, in the course of development, a definite accumulation of mitochondria occurs in certain areas ; on the other, there are eggs,
and among these the sea urchin eggs, in which no such accumulation
occurs. Evidently, if the mitochondria are to play a significant role in
morphogenesis, one would expect their differential distribution to be a
ALBERTO MONROY AND RACHELE MAGGIO
confine ourselves to mentioning very briefly some of the most pertinent
data. The most extensive studies on this subject have been made by
Reverberi and his collaborators on the ascidians (summarized in
Reverberi, 1961). Immediately after fertilization mitochondria accumulate at the vegetal pole of the egg, then in the yellow crescent and
finally in the primitive muscular cells and in the musculature of the
swimming larva (Reverberi, 1957). By the use of different inhibitors
Reverberi and his collaborators were then able to show that the
inhibition of mitochondrial enzymes results in alterations in the
territory where the mitochondria are accumulated. Although the interpretation of these observations is difficult, on the basis of available
evidence they certainly suggest that the differential accumulation of
mitochondria in the various territories of the embryo may play an
important morphogenetic role. Similar results have been obtained by a
number of other investigators on other eggs. The unequal distribution
of cytochrome oxidase among the different blastomeres of the Ascidian
embryo has been confirmed by Berg (1956).
The question of the gradient distribution of mitochondria in the sea
urchin embryo and its significance is discussed by Lallier (1964). In
this context it will only be mentioned that, according to Gustaf son and
Lenicque (1952, 1955), until the late blastula stage the mitochondria are
uniformly distributed throughout the embryo but at the mesenchyme
blastula stage an animal-vegetal gradient appears which becomes more
and more evident as development proceeds. However, the technique
used by Gustafson and Lenicque for counting mitochondria has been
questioned by Shaver (1956, 1957) who could not confirm their results.
More recently the problem of the gradient distribution of mitochondria
has been submitted to a careful electron microscopic analysis by Berg
et al. (1962) who have also been unable to obtain evidence of the
existence of such a gradient in the sea urchin blastula. On the other hand
they could detect a certain accumulation of yolk platelets in the vegetal
region of the embryo and in the cells of the primary mesenchyme. Some
of the observations of Gustafson and Lenicque were confirmed by
Shaver (1956,1957), e.g., the accumulation of mitochondria close to the
ciliated border and an increase of the mitochondria in the cells of the gut
and in the cells of the developing arms of the pluteus. Whether such
accumulations have a morphogenetic significance is doubtful.
The present situation is therefore quite perplexing. On the one hand
we find eggs in which, in the course of development, a definite accumulation of mitochondria occurs in certain areas ; on the other, there are eggs,
and among these the sea urchin eggs, in which no such accumulation
occurs. Evidently, if the mitochondria are to play a significant role in
morphogenesis, one would expect their differential distribution to be a