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neurulation), and is strictly limited to the nervous system. An Ascidian
tadpole is not very dissimilar from an Amphibian one, so that it might
be supposed that the enzyme would appear late in development, and in
the neural system. However, in Ciona, the cytochemical reaction of
Koelle shows very clearly that this supposition is wrong; no trace of the
enzyme is found in the early stages of development. The first evidence
of its presence can be noticed at neurulation, not in the neural plate, but
F I G . 17. Localization of Cholinesterase in different stages of development of Ciona.
a and 6, neurula stages; the enzyme is localized in the presumptive muscular cells;
c, embryo at the tail-bud stage; d, a swimming tadpole; e, a tadpole in metamorphosis
(Durante, 1956).
on the contrary, localized in the muscular territory (Fig. 17). In the
swimming tadpole the enzyme is also localized in the muscles (of the tail).
Like the alkaline phosphatases and dopa oxidase, Cholinesterase is an
enzyme which appears (according to cytochemical reactions) very late
in embryonic development, and certainly is not correlated with the
determination of the musculature, since this territory, as can be checked
by isolation experiments, is already determined at the 8-cell stage.
In contrast to dopa oxidase, the synthesis of Cholinesterase is not