ANIMALIZATION AND VEGETALIZATION
171
VII. Metabolism of Nucleotides and Ribonucleic Acid
The ribonucleic acid content of eggs remains essentially constant
throughout development according to Schmidt et al. (1948), Villee et al.
(1949), and Agrell and Persson (1956). But Elson et al. (1954) have
observed fluctuations in the ribonucleic acid content, which drops
sharply just after fertilization, followed by a first rise during segmentation and a second rise just before the onset of gastrulation. Bäckström
(1959c) has studied the quantitative changes of ribonucleic acid and has
observed a decrease in content during development. The ribonucleic acid
content shows fluctuations which would correspond to the predominance
either of synthesis or of degradation of ribonucleic acid. Several periods
when synthesis is in excess have been detected ; they occur during the
early blastula stage, at hatching, and in connection with two different
phases of gastrulation. Comparison between the quantitative variations
of ribonucleic acid content and the activity curve of glucose-6-phosphate
dehydrogenase, an enzyme of the hexosemonophosphate shunt, led
Bäckström to suggest that the periods of predominant synthesis of
ribonucleic acid correspond closely to those of enhanced glucose-6phosphate dehydrogenase activity. The ribonucleic acid would be
synthesized from the pentose phosphate formed by hexosemonophosphate shunt activity.
Bäckström has extended this research to the study of animalized and
vegetalized embryos. In the embryos animalized by iodosobenzoic acid
the ribonucleic acid content decreases less quickly than in the controls.
Bäckström explains this phenomenon as the result of an excess in the
synthesis of the ribonucleic acid in connection with the increase in
activity of the hexosemonophosphate shunt during animalization. The
ribonucleic acid content of vegetalized larvae also exceeds that of the
normal larvae although the activity of the shunt in the vegetalized
embryos is lower than in the normal larvae. Nevertheless, this observation is not incompatible with Bäckström's interpretation of a relation
between shunt activity and ribonucleic acid synthesis. The analogous
quantitative evolution of the ribonucleic acid content in the animalized
and the vegetalized embryos may depend either upon an increase in
synthesis or upon the inhibition of the breakdown of ribonucleic acid.
Inhibition of breakdown by lithium could then account for the high
content of ribonucleic acid in vegetalized embryos. Finally it must be
added that it is specially important to establish that the changes in
ribonucleic acid content are a function of animalization and of vegetalization, irrespective of the way in which animalization and vegetalization
are brought about. The study of the ribonucleic acid changes in embryos
171
VII. Metabolism of Nucleotides and Ribonucleic Acid
The ribonucleic acid content of eggs remains essentially constant
throughout development according to Schmidt et al. (1948), Villee et al.
(1949), and Agrell and Persson (1956). But Elson et al. (1954) have
observed fluctuations in the ribonucleic acid content, which drops
sharply just after fertilization, followed by a first rise during segmentation and a second rise just before the onset of gastrulation. Bäckström
(1959c) has studied the quantitative changes of ribonucleic acid and has
observed a decrease in content during development. The ribonucleic acid
content shows fluctuations which would correspond to the predominance
either of synthesis or of degradation of ribonucleic acid. Several periods
when synthesis is in excess have been detected ; they occur during the
early blastula stage, at hatching, and in connection with two different
phases of gastrulation. Comparison between the quantitative variations
of ribonucleic acid content and the activity curve of glucose-6-phosphate
dehydrogenase, an enzyme of the hexosemonophosphate shunt, led
Bäckström to suggest that the periods of predominant synthesis of
ribonucleic acid correspond closely to those of enhanced glucose-6phosphate dehydrogenase activity. The ribonucleic acid would be
synthesized from the pentose phosphate formed by hexosemonophosphate shunt activity.
Bäckström has extended this research to the study of animalized and
vegetalized embryos. In the embryos animalized by iodosobenzoic acid
the ribonucleic acid content decreases less quickly than in the controls.
Bäckström explains this phenomenon as the result of an excess in the
synthesis of the ribonucleic acid in connection with the increase in
activity of the hexosemonophosphate shunt during animalization. The
ribonucleic acid content of vegetalized larvae also exceeds that of the
normal larvae although the activity of the shunt in the vegetalized
embryos is lower than in the normal larvae. Nevertheless, this observation is not incompatible with Bäckström's interpretation of a relation
between shunt activity and ribonucleic acid synthesis. The analogous
quantitative evolution of the ribonucleic acid content in the animalized
and the vegetalized embryos may depend either upon an increase in
synthesis or upon the inhibition of the breakdown of ribonucleic acid.
Inhibition of breakdown by lithium could then account for the high
content of ribonucleic acid in vegetalized embryos. Finally it must be
added that it is specially important to establish that the changes in
ribonucleic acid content are a function of animalization and of vegetalization, irrespective of the way in which animalization and vegetalization
are brought about. The study of the ribonucleic acid changes in embryos