68
G . RE VERB ERI
'conditioned' by other territories, in fact, if at the 8-cell stage one
separates the 4 anterior blastomeres (which contain the territory of the
brain) from the 4 posterior ones (which contain the territory of the
muscles), one can observe Cholinesterase in the posterior hemi-embryos
(Durante, 1957). The synthesis of the enzyme, then, seems to be autonomous : certainly it is autonomous from the nervous system.
Not all the Tunicates, however, show Cholinesterase localized in the
musculature as does Ciona. In Clavelina lepadiformis, whose development was also followed step by step (Durante, 1959), the Cholinesterase,
which also appears only at neurulation, is localized in the neural plate,
as in the Amphibians. In the swimming larva, Cholinesterase is to be
found only in the brain and spinal chord; no trace of it is found in the
musculature.
In the Appendiculariae, which at the adult stage look like a larva of
Ciona and Clavelina, the Cholinesterase is localized both in the
musculature and in the nervous system!
How can such a different localization of the same enzyme in these
similar forms be explained? One can only make suggestions. The larvae
of Ciona are very active, they move continuously and rapidly, the
direction of movement, also, varies continuously and they are able to
swim for a day or more: it seems as if their principal function was
movement. The larvae of Clavelina, on the contrary, do not show (in the
laboratory) such active movement; they have a tail with a large fin;
possibly the tail is more adapted for floating than for autonomous
movement. The Appendiculariae also possess a tail with a large fin, the
tail being stronger than in the Clavelina tadpoles, and apparently
adapted for swimming and for floating. The presence of Cholinesterase
in the tail (Ciona), or in the brain (Clavelina lepadiformis), or in the tail
and in the brain (Appendiculariae), is possibly correlated with such
various kinds of motility.
III. The Biological Significance of the Egg Constituents
What one would like to know when considering the egg constituents
is their biological significance. No problem is more difficult to solve than
this but it has been pursued in three different ways: A. by studying the
quantitative variations of certain constituents during development;
B. by specifically inhibiting enzymatic activities; C. by performing
heterospecific hybridizations.
A. Quantitative Variations of Certain Egg Constituents during Development
At the present moment knowledge of these variations is more a need
than a reality, however, studies on the variations of some enzymes
G . RE VERB ERI
'conditioned' by other territories, in fact, if at the 8-cell stage one
separates the 4 anterior blastomeres (which contain the territory of the
brain) from the 4 posterior ones (which contain the territory of the
muscles), one can observe Cholinesterase in the posterior hemi-embryos
(Durante, 1957). The synthesis of the enzyme, then, seems to be autonomous : certainly it is autonomous from the nervous system.
Not all the Tunicates, however, show Cholinesterase localized in the
musculature as does Ciona. In Clavelina lepadiformis, whose development was also followed step by step (Durante, 1959), the Cholinesterase,
which also appears only at neurulation, is localized in the neural plate,
as in the Amphibians. In the swimming larva, Cholinesterase is to be
found only in the brain and spinal chord; no trace of it is found in the
musculature.
In the Appendiculariae, which at the adult stage look like a larva of
Ciona and Clavelina, the Cholinesterase is localized both in the
musculature and in the nervous system!
How can such a different localization of the same enzyme in these
similar forms be explained? One can only make suggestions. The larvae
of Ciona are very active, they move continuously and rapidly, the
direction of movement, also, varies continuously and they are able to
swim for a day or more: it seems as if their principal function was
movement. The larvae of Clavelina, on the contrary, do not show (in the
laboratory) such active movement; they have a tail with a large fin;
possibly the tail is more adapted for floating than for autonomous
movement. The Appendiculariae also possess a tail with a large fin, the
tail being stronger than in the Clavelina tadpoles, and apparently
adapted for swimming and for floating. The presence of Cholinesterase
in the tail (Ciona), or in the brain (Clavelina lepadiformis), or in the tail
and in the brain (Appendiculariae), is possibly correlated with such
various kinds of motility.
III. The Biological Significance of the Egg Constituents
What one would like to know when considering the egg constituents
is their biological significance. No problem is more difficult to solve than
this but it has been pursued in three different ways: A. by studying the
quantitative variations of certain constituents during development;
B. by specifically inhibiting enzymatic activities; C. by performing
heterospecific hybridizations.
A. Quantitative Variations of Certain Egg Constituents during Development
At the present moment knowledge of these variations is more a need
than a reality, however, studies on the variations of some enzymes