I I . T H E
E M B R Y O L O G Y
O F
A S C I D I A N S
65
The precocious enzymatic segregation, however, must not be assumed
to be a general occurrence in embryological development; very often
the segregation takes place very late in development as in the case of
alkaline phosphatase, tyrosinase and Cholinesterase.
(b) The embryonic history of the alkaline phosphatases has been
followed by Minganti (1954). It is not possible, in the unsegmented egg
or in the first stages of development, to reveal by specific cytochemical
reaction the presence of any phosphatases. One cannot deny their
existence, but at the most very few molecules can be present. At
neurulation, however, there occurs a spectacular production of these
F I G S . 14, 15. Alkaline phosphatase during development (Ascidiella asperea). T h e
enzyme appears in the entoderm of the neurula (Fig. 14), and in the intestine of the
tadpole (Fig. 15) (Minganti, 1954).
enzymes; moreover, they appear to be localized only in the territory of
the primitive intestine, which is about to differentiate (Fig. 14). The
enzymes are particularly abundant in the lateral and ventral parts of
the posterior intestine. In the swimming larva also, the alkaline phosphatases are strictly localized in the intestine (Fig. 15) but after metamorphosis the enzymes can be found in the stomach. The territory from
which the intestine arises is irrevocably determined very early; at the
64-cell stage, the entodermal territory is already segregated from other
territories, but no trace of phosphatases can be found, at that stage, in
its cells. The enzymes, then, do not impose the 'entodermal' character
on a cell, on the contrary, their synthesis takes place in cells which are
already entodermal.
(c) Another enzyme whose embryonic history has been followed is
dopa oxidase (Minganti, 1951a).
E M B R Y O L O G Y
O F
A S C I D I A N S
65
The precocious enzymatic segregation, however, must not be assumed
to be a general occurrence in embryological development; very often
the segregation takes place very late in development as in the case of
alkaline phosphatase, tyrosinase and Cholinesterase.
(b) The embryonic history of the alkaline phosphatases has been
followed by Minganti (1954). It is not possible, in the unsegmented egg
or in the first stages of development, to reveal by specific cytochemical
reaction the presence of any phosphatases. One cannot deny their
existence, but at the most very few molecules can be present. At
neurulation, however, there occurs a spectacular production of these
F I G S . 14, 15. Alkaline phosphatase during development (Ascidiella asperea). T h e
enzyme appears in the entoderm of the neurula (Fig. 14), and in the intestine of the
tadpole (Fig. 15) (Minganti, 1954).
enzymes; moreover, they appear to be localized only in the territory of
the primitive intestine, which is about to differentiate (Fig. 14). The
enzymes are particularly abundant in the lateral and ventral parts of
the posterior intestine. In the swimming larva also, the alkaline phosphatases are strictly localized in the intestine (Fig. 15) but after metamorphosis the enzymes can be found in the stomach. The territory from
which the intestine arises is irrevocably determined very early; at the
64-cell stage, the entodermal territory is already segregated from other
territories, but no trace of phosphatases can be found, at that stage, in
its cells. The enzymes, then, do not impose the 'entodermal' character
on a cell, on the contrary, their synthesis takes place in cells which are
already entodermal.
(c) Another enzyme whose embryonic history has been followed is
dopa oxidase (Minganti, 1951a).