AMPHIBIAN AND INVERTEBRATE DEVELOPMENT 63
results are interpreted in terms of ecological and systematic morphological knowledge.
II. Embryonic Development and Metamorphosis of Amphibians
Fertilization initiates the process by which the egg gradually develops
into an embryo and then into a larva. From the point of view of energy
supplies, the embryo is a closed system or 'closed box' (Needham, 1931,
1942) in which destructive and constructive activities occur without
resort to outside resources of energy. The yolk reserves provide the
necessary energy for development, and oxygen, water, and mineral salts
are the only external substances utilized by the embryo.
Faure-Fremiet (1925) described embryonic development as a phenomenon of 'croissance autophagique', eloquently expressing that the
organism feeds on itself; in other words it utilizes the deutoplasmic
reserves accumulated during vitellogenesis.
Comparison of the protein, carbohydrate and lipid contents of the
mature egg with those after complete utilization of the yolk reveals
that these compounds are oxidized to produce the energy necessary for
development.
Our data on Bufo vulgaris and Rana esculenta indicate that in this
period carbohydrates and lipids are reduced by 40-50%. Total nitrogen
is reduced by only 16-18% (Urbani, 1955a, b ; 1957). We shall revert to
these data in greater detail when dealing with energy resources and
their sequence of utilization during development.
These autophagic 'closed box' conditions last until the yolk reserves
have been used up completely. We agree with Cotronei (1922, 1930,
1932), that the larval stage begins immediately after complete resorption of the yolk. Cotronei has proved that the transition from embryonic to larval life is accompanied by metabolic and humoral changes
in the organism which, no longer energetically self sufficient, starts
feeding.
Our enzymological studies give evidence of clearly defined enzymatic
activity at this transitional stage, which is a first indication of the close
interrelation of the biological sequences, which morphologically correspond to the characteristic stages of development, and enzymatic and
biochemical behaviour in general.
Among the phenomena which characterize embryonic growth are
those of hydration: the embryo's increase in weight is essentially due
to water uptake (Bialaszewicz, 1912; Briggs, 1939; Brown, 1941;
Davenport, 1897; Dempster, 1933;Ranzi, 1941; Ruffini, 1925; Schaper,
1902).
Fig. 1 shows the fresh and reduced (see Zeuthen, 1948) weight curves
of an embryo of Bufo vulgaris: dry weight is reduced by 23%, from
results are interpreted in terms of ecological and systematic morphological knowledge.
II. Embryonic Development and Metamorphosis of Amphibians
Fertilization initiates the process by which the egg gradually develops
into an embryo and then into a larva. From the point of view of energy
supplies, the embryo is a closed system or 'closed box' (Needham, 1931,
1942) in which destructive and constructive activities occur without
resort to outside resources of energy. The yolk reserves provide the
necessary energy for development, and oxygen, water, and mineral salts
are the only external substances utilized by the embryo.
Faure-Fremiet (1925) described embryonic development as a phenomenon of 'croissance autophagique', eloquently expressing that the
organism feeds on itself; in other words it utilizes the deutoplasmic
reserves accumulated during vitellogenesis.
Comparison of the protein, carbohydrate and lipid contents of the
mature egg with those after complete utilization of the yolk reveals
that these compounds are oxidized to produce the energy necessary for
development.
Our data on Bufo vulgaris and Rana esculenta indicate that in this
period carbohydrates and lipids are reduced by 40-50%. Total nitrogen
is reduced by only 16-18% (Urbani, 1955a, b ; 1957). We shall revert to
these data in greater detail when dealing with energy resources and
their sequence of utilization during development.
These autophagic 'closed box' conditions last until the yolk reserves
have been used up completely. We agree with Cotronei (1922, 1930,
1932), that the larval stage begins immediately after complete resorption of the yolk. Cotronei has proved that the transition from embryonic to larval life is accompanied by metabolic and humoral changes
in the organism which, no longer energetically self sufficient, starts
feeding.
Our enzymological studies give evidence of clearly defined enzymatic
activity at this transitional stage, which is a first indication of the close
interrelation of the biological sequences, which morphologically correspond to the characteristic stages of development, and enzymatic and
biochemical behaviour in general.
Among the phenomena which characterize embryonic growth are
those of hydration: the embryo's increase in weight is essentially due
to water uptake (Bialaszewicz, 1912; Briggs, 1939; Brown, 1941;
Davenport, 1897; Dempster, 1933;Ranzi, 1941; Ruffini, 1925; Schaper,
1902).
Fig. 1 shows the fresh and reduced (see Zeuthen, 1948) weight curves
of an embryo of Bufo vulgaris: dry weight is reduced by 23%, from