66
ENRICO URBANI
end of weight increase; II—still feeding larva with grown hind limbs;
III—larva with left fore limb protruding from the operculum; IV—
larva with the four limbs free and regressive tail; V—the animal 1-2
days after complete metamorphosis.
These same stages were chosen for our studies on the biochemical
aspects of metamorphosis and are shown as such in the figures. The
cytological observations of the larvae of Triton and Rana during metamorphosis reveal cellular shrinkage more at the expense of the cytoplasm than of the nucleus. More appreciable changes are found in the
connective tissue, where the fibrils appear to scatter and regroup themselves somewhat irregularly (Cotronei, 1914, 1920). Variations in vitamin C content have been observed (Bucci, 1951). Medola et al. (1956)
working on the participation of vitamins in metamorphosis, established
that metamorphosis is inhibited by penicillin, which destroys the intestinal flora from which the larva draws its vitamin supply.
We do not know which factors initiate metamorphosis in the mature
larva: the phenomenon is probably due to a combination of factors such
as age, weight, feeding and environmental temperature (Adolph, 1931;
Allen, 1919; Etkin, 1932, 1955; Schreiber, 1934). In relation to larval
age, Gudernatsch (1912) and Cotronei (1913, 1914) have emphasized
the role played by thyroxin, showing that thyroid administration to the
larva reduces the period required for the larval stage. Thyroid and
glycocorticoids are believed to play a primary role in mobilizing
proteins (Frieden, 1956; Frieden and Naile, 1955; Withe, 1949).
Great importance is attached to the complex phenomena of tissue
destruction and reconstruction, which may bring about marked
changes, ecologically and morphologically, in the amphibian after
metamorphosis.
These processes of destruction, resorption and reconstruction affecting particularly the mouth, branchiae, digestive apparatus and tail
are accompanied by phagocytosis (Guiweisse, 1905; Mercier, 1906;
Metchnikoif, 1883), but in some organs, e.g. in the operculum, phagocytic invasion occurs after histolysis (Helff, 1926). It is quite possible
that phagocytes have among their functions that of producing proteases
for extracellular activities or of stimulating the cells to produce intracellular proteolytic enzymes (cathepsins).
In the processes of histolysis and resorption—the most remarkable
being perhaps tail regression in the Anura—a primary role is certainly
played by proteolytic enzymes, as will be shown later. The series of
events which take place at metamorphosis are shown not only in the
amphibians, but also in holometabolous insects (Russo-Caia, 1960a).
This may be adduced from the U-shaped respiratory curve (Petrucci,
1958; Taranta, 1947), which is entirely different from the curves
ENRICO URBANI
end of weight increase; II—still feeding larva with grown hind limbs;
III—larva with left fore limb protruding from the operculum; IV—
larva with the four limbs free and regressive tail; V—the animal 1-2
days after complete metamorphosis.
These same stages were chosen for our studies on the biochemical
aspects of metamorphosis and are shown as such in the figures. The
cytological observations of the larvae of Triton and Rana during metamorphosis reveal cellular shrinkage more at the expense of the cytoplasm than of the nucleus. More appreciable changes are found in the
connective tissue, where the fibrils appear to scatter and regroup themselves somewhat irregularly (Cotronei, 1914, 1920). Variations in vitamin C content have been observed (Bucci, 1951). Medola et al. (1956)
working on the participation of vitamins in metamorphosis, established
that metamorphosis is inhibited by penicillin, which destroys the intestinal flora from which the larva draws its vitamin supply.
We do not know which factors initiate metamorphosis in the mature
larva: the phenomenon is probably due to a combination of factors such
as age, weight, feeding and environmental temperature (Adolph, 1931;
Allen, 1919; Etkin, 1932, 1955; Schreiber, 1934). In relation to larval
age, Gudernatsch (1912) and Cotronei (1913, 1914) have emphasized
the role played by thyroxin, showing that thyroid administration to the
larva reduces the period required for the larval stage. Thyroid and
glycocorticoids are believed to play a primary role in mobilizing
proteins (Frieden, 1956; Frieden and Naile, 1955; Withe, 1949).
Great importance is attached to the complex phenomena of tissue
destruction and reconstruction, which may bring about marked
changes, ecologically and morphologically, in the amphibian after
metamorphosis.
These processes of destruction, resorption and reconstruction affecting particularly the mouth, branchiae, digestive apparatus and tail
are accompanied by phagocytosis (Guiweisse, 1905; Mercier, 1906;
Metchnikoif, 1883), but in some organs, e.g. in the operculum, phagocytic invasion occurs after histolysis (Helff, 1926). It is quite possible
that phagocytes have among their functions that of producing proteases
for extracellular activities or of stimulating the cells to produce intracellular proteolytic enzymes (cathepsins).
In the processes of histolysis and resorption—the most remarkable
being perhaps tail regression in the Anura—a primary role is certainly
played by proteolytic enzymes, as will be shown later. The series of
events which take place at metamorphosis are shown not only in the
amphibians, but also in holometabolous insects (Russo-Caia, 1960a).
This may be adduced from the U-shaped respiratory curve (Petrucci,
1958; Taranta, 1947), which is entirely different from the curves