EARLY DEVELOPMENT OF THE SEA URCHIN
137
he had used for the preparation of his material is likely to lead to
autolytic artifacts which may bias the whole picture. As previously
mentioned (Section II), this may be especially serious in those stages
where proteolytic activity is known to be high, such as in the first few
minutes following fertilization (Lundblad, 1950; Maggio, 1957) or at the
blastula stage. It may be pertinent to quote here some observations made
by Monroy et al. (1961a) in the teleostean fish Oryzias latipes. By
administering radioactive amino-acids to egg-laying females, eggs were
obtained in which 90 per cent of the radioactivity was in the yolk in a
protein-bound condition (precipitable by trichloroacetic acid). Very
little, if any, utilization of yolk takes place until the late gastrula stageThen almost all at once a considerable amount of radioactivity appears
in low molecular form (soluble in trichloroacetic acid) and, at nearly the
same time, incorporation into the embryonic proteins begins. Interestingly enough, a fresh breakdown of yolk occurs only after the first low
molecular pool has been exhausted. This suggests that the low molecular
pool stays between the yolk and the embryonic proteins. It was also
found that, in the pool, by far the highest radioactivity is present in a
fairly large peptide whereas the radioactivity of the free amino-acid
is quite low. This may indicate that the peptide is an important
intermediate. It must be stressed that the great advantage of the
teleostean egg is that the yolk can easily be separated from the embryo,
which is not the case in the sea urchin.
Using a quantitative immunological technique, Perlmann and
Kaltenbach (1957) found a quantitative decrease of two antigens,
especially during gastrulation, and they interpreted this as an indication
of the breakdown of the yolk proteins. The fact is that the data so far
available indicate that the composition of the amino-acid and peptide
pool of the egg does not change much in the course of early development
(Kavanau, 1954) and since protein synthesis is still going on, this means
that the pool must somehow be being replenished and the yolk seems
the most likely candidate for this function. Admittedly, there is no proof
that other proteins, too, may not participate in this. The study of the
respiratory quotient does not tell much about yolk utilization although
the available data (see Section III, A) are indicative of protein and,
possibly, lipid utilization from the beginning of development.
An interesting problem is the form in which the breakdown products
of yolk are utilized by the embryo for its protein synthesis. The presence
of peptides in the pool has been recognized at all stages of development
and, as far as can be judged from the available data, they do not seem to
undergo significant quantitative changes. Nothing is known of their
role in the process of embryonic synthesis. In unfertilized eggs labelled
in the ovary with
35
S-methionine, free glutathione appears to take up a
137
he had used for the preparation of his material is likely to lead to
autolytic artifacts which may bias the whole picture. As previously
mentioned (Section II), this may be especially serious in those stages
where proteolytic activity is known to be high, such as in the first few
minutes following fertilization (Lundblad, 1950; Maggio, 1957) or at the
blastula stage. It may be pertinent to quote here some observations made
by Monroy et al. (1961a) in the teleostean fish Oryzias latipes. By
administering radioactive amino-acids to egg-laying females, eggs were
obtained in which 90 per cent of the radioactivity was in the yolk in a
protein-bound condition (precipitable by trichloroacetic acid). Very
little, if any, utilization of yolk takes place until the late gastrula stageThen almost all at once a considerable amount of radioactivity appears
in low molecular form (soluble in trichloroacetic acid) and, at nearly the
same time, incorporation into the embryonic proteins begins. Interestingly enough, a fresh breakdown of yolk occurs only after the first low
molecular pool has been exhausted. This suggests that the low molecular
pool stays between the yolk and the embryonic proteins. It was also
found that, in the pool, by far the highest radioactivity is present in a
fairly large peptide whereas the radioactivity of the free amino-acid
is quite low. This may indicate that the peptide is an important
intermediate. It must be stressed that the great advantage of the
teleostean egg is that the yolk can easily be separated from the embryo,
which is not the case in the sea urchin.
Using a quantitative immunological technique, Perlmann and
Kaltenbach (1957) found a quantitative decrease of two antigens,
especially during gastrulation, and they interpreted this as an indication
of the breakdown of the yolk proteins. The fact is that the data so far
available indicate that the composition of the amino-acid and peptide
pool of the egg does not change much in the course of early development
(Kavanau, 1954) and since protein synthesis is still going on, this means
that the pool must somehow be being replenished and the yolk seems
the most likely candidate for this function. Admittedly, there is no proof
that other proteins, too, may not participate in this. The study of the
respiratory quotient does not tell much about yolk utilization although
the available data (see Section III, A) are indicative of protein and,
possibly, lipid utilization from the beginning of development.
An interesting problem is the form in which the breakdown products
of yolk are utilized by the embryo for its protein synthesis. The presence
of peptides in the pool has been recognized at all stages of development
and, as far as can be judged from the available data, they do not seem to
undergo significant quantitative changes. Nothing is known of their
role in the process of embryonic synthesis. In unfertilized eggs labelled
in the ovary with
35
S-methionine, free glutathione appears to take up a