AMPHIBIAN AND INVERTEBRATE DEVELOPMENT 99
some of them are formed only at subsequent stages. Our knowledge of
enzymatic properties is not sufficient to provide a precise answer, and
is complicated by the confusion between the enzymatic molecule considered as a chemical entity and enzymatic activity as displayed by the
molecule itself in vivo. An enzyme, in fact, may be present, but inactive.
At times, different research techniques enable us to detect a previously
ignored enzyme at given stages of development (Spirito, 1934).
The eggs of amphibians and invertebrates have been shown to contain proteases, amylases, and lipases (as proved by the splitting of the
respective substrates); their activity grows more intensive or weakens
during development. On the whole, it can be said that preformation
prevails, and that further evolution of the enzymes results from the
activation and inhibition of the functions of preexisting molecules and
of their multiplication.
It was also seen that some enzymes, e.g. alkaline proteinases and
digestive amylases, display their maximal activity at given developmental stages, showing biochemical as well as histological differentiations. With regard to these two enzymes, it is doubtful whether we
should speak of true epigenesis or of a more intensive activity; it is
possible that the techniques used are not sensitive enough to reveal
their presence at earlier developmental stages. Studies on alkaline proteinase in Musca domestica L. show also that its activity ceases during
metamorphosis.
These considerations point to three possibilities: all the enzymes are
present and active in the egg (even though their activity, being minimal,
cannot be definitely ascertained), and increase in activity during development; or some enzymatic molecules come into play only at given
stages; or, lastly, some of the enzymatic molecules are formed ex novo.
These possibilities are all acceptable; a satisfactory solution to the
problem will only be made possible by an objective examination of the
data available and further research (ten Cate, 1952).
Spiegelman and Steinbach (1945) studied oxygen uptake in whole
embryos of Rana pipiens, and in embryonic homogenates: the values
were found to be low in whole embryo at the initial stages of development, and high in the homogenate; the situation is then reversed in the
tail bud stage. The authors attribute these results to the structural
orientation and molecular geometry of enzymes and substrates, destroyed
and modified by homogenization and resulting, at the different stages
of development, in an increase or inhibition of enzymatic acitivity.
Similar results were obtained much earlier by Brächet (1934), who drew
the same conclusions from his experiments. Recently Gregg (1960) repeated the studies of Brächet (1934) and Spiegelman and Steinbach
(1945) and drew similar conclusions. Mention should be made, in this
some of them are formed only at subsequent stages. Our knowledge of
enzymatic properties is not sufficient to provide a precise answer, and
is complicated by the confusion between the enzymatic molecule considered as a chemical entity and enzymatic activity as displayed by the
molecule itself in vivo. An enzyme, in fact, may be present, but inactive.
At times, different research techniques enable us to detect a previously
ignored enzyme at given stages of development (Spirito, 1934).
The eggs of amphibians and invertebrates have been shown to contain proteases, amylases, and lipases (as proved by the splitting of the
respective substrates); their activity grows more intensive or weakens
during development. On the whole, it can be said that preformation
prevails, and that further evolution of the enzymes results from the
activation and inhibition of the functions of preexisting molecules and
of their multiplication.
It was also seen that some enzymes, e.g. alkaline proteinases and
digestive amylases, display their maximal activity at given developmental stages, showing biochemical as well as histological differentiations. With regard to these two enzymes, it is doubtful whether we
should speak of true epigenesis or of a more intensive activity; it is
possible that the techniques used are not sensitive enough to reveal
their presence at earlier developmental stages. Studies on alkaline proteinase in Musca domestica L. show also that its activity ceases during
metamorphosis.
These considerations point to three possibilities: all the enzymes are
present and active in the egg (even though their activity, being minimal,
cannot be definitely ascertained), and increase in activity during development; or some enzymatic molecules come into play only at given
stages; or, lastly, some of the enzymatic molecules are formed ex novo.
These possibilities are all acceptable; a satisfactory solution to the
problem will only be made possible by an objective examination of the
data available and further research (ten Cate, 1952).
Spiegelman and Steinbach (1945) studied oxygen uptake in whole
embryos of Rana pipiens, and in embryonic homogenates: the values
were found to be low in whole embryo at the initial stages of development, and high in the homogenate; the situation is then reversed in the
tail bud stage. The authors attribute these results to the structural
orientation and molecular geometry of enzymes and substrates, destroyed
and modified by homogenization and resulting, at the different stages
of development, in an increase or inhibition of enzymatic acitivity.
Similar results were obtained much earlier by Brächet (1934), who drew
the same conclusions from his experiments. Recently Gregg (1960) repeated the studies of Brächet (1934) and Spiegelman and Steinbach
(1945) and drew similar conclusions. Mention should be made, in this