II.
THE EMBRYOLOGY OF ASCIDIANS
61
reach the region of the yellow crescent. We shall see later what this
peculiar segregation might mean.
Another concentration of RNA is observed around the nuclei of the
blastomeres (Fig. 4). This concentration can be explained by assuming
that the newly synthesized RNA is liberated at every rupture of the
nucleus during mitosis or by assuming that the granules of RNA,
connected with the endoplasmic reticulum, become 'mobilized' at every
mitotic cycle. In either case the presence of RNA where the proteins of
the spindle will be synthesized may be necessary.
The constituents of the yellow-crescent are segregated during
development into the posterior vegetal blastomeres; thus, the RNA of
the yellow crescent will also be accumulated in these blastomeres
(Figs. 5 and 6). Since these give rise to the muscles, the peculiar,
precocious segregation of the original RNA content of the egg might be
a device to assure a proper synthesis of muscular proteins. If new RNA
is synthesized continuously during development, it certainly does not
occur at the same rate in the different cells. Generally cells which display
much activity in protein synthesis, or in multiplication, are rich in RNA.
In the developing embryo of Ascidians, tissues differentiate at
different times, and therefore every tissue has its own time of activity;
the maximum differentiation or activity of the different tissues corresponds to their maximum content of RNA. This is evident in the neurula
stage, where neural and mesenchymal tissues are especially rich in RNA
(Figs. 7, 8 and 9). These tissues are also the most active at this stage,
since the neural tissue is undergoing differentiation, and the mesenchymal tissue is initiating a wave of spectacular multiplication. A
similar richness in RNA was observed earlier, also by means of histochemical reactions, in the neural plate of Amphibians (Brächet, 1941).
In the tadpoles of the Ascidians, the cells which are richest in RNA are
those of the adhesive papillae; they are also the most active; in fact,
they provide for the secretion of the substance which will allow the
tadpoles to attach themselves to the substratum, and initiate metamorphosis.
In conclusion, the facts reported seem to provide embryological
support in favour of the hypothesis that RNA is, somehow, concerned
with protein synthesis (Caspersson, 1941; Brächet, 1942).
2. Deoxyribonucleic Acid
The content of deoxyribonucleic acid (DNA) of the different nuclei of
the developing egg has also been investigated cytochemically (Mancuso,
1959b). The disputed question is whether this content is absolutely
constant, as many authors believe (cf. Pasteéis and Lison, 1955).
DNA is a nuclear constituent; Grosch (1958), however, maintains
THE EMBRYOLOGY OF ASCIDIANS
61
reach the region of the yellow crescent. We shall see later what this
peculiar segregation might mean.
Another concentration of RNA is observed around the nuclei of the
blastomeres (Fig. 4). This concentration can be explained by assuming
that the newly synthesized RNA is liberated at every rupture of the
nucleus during mitosis or by assuming that the granules of RNA,
connected with the endoplasmic reticulum, become 'mobilized' at every
mitotic cycle. In either case the presence of RNA where the proteins of
the spindle will be synthesized may be necessary.
The constituents of the yellow-crescent are segregated during
development into the posterior vegetal blastomeres; thus, the RNA of
the yellow crescent will also be accumulated in these blastomeres
(Figs. 5 and 6). Since these give rise to the muscles, the peculiar,
precocious segregation of the original RNA content of the egg might be
a device to assure a proper synthesis of muscular proteins. If new RNA
is synthesized continuously during development, it certainly does not
occur at the same rate in the different cells. Generally cells which display
much activity in protein synthesis, or in multiplication, are rich in RNA.
In the developing embryo of Ascidians, tissues differentiate at
different times, and therefore every tissue has its own time of activity;
the maximum differentiation or activity of the different tissues corresponds to their maximum content of RNA. This is evident in the neurula
stage, where neural and mesenchymal tissues are especially rich in RNA
(Figs. 7, 8 and 9). These tissues are also the most active at this stage,
since the neural tissue is undergoing differentiation, and the mesenchymal tissue is initiating a wave of spectacular multiplication. A
similar richness in RNA was observed earlier, also by means of histochemical reactions, in the neural plate of Amphibians (Brächet, 1941).
In the tadpoles of the Ascidians, the cells which are richest in RNA are
those of the adhesive papillae; they are also the most active; in fact,
they provide for the secretion of the substance which will allow the
tadpoles to attach themselves to the substratum, and initiate metamorphosis.
In conclusion, the facts reported seem to provide embryological
support in favour of the hypothesis that RNA is, somehow, concerned
with protein synthesis (Caspersson, 1941; Brächet, 1942).
2. Deoxyribonucleic Acid
The content of deoxyribonucleic acid (DNA) of the different nuclei of
the developing egg has also been investigated cytochemically (Mancuso,
1959b). The disputed question is whether this content is absolutely
constant, as many authors believe (cf. Pasteéis and Lison, 1955).
DNA is a nuclear constituent; Grosch (1958), however, maintains