II. THE EMBRYOLOGY OF ASCIDIANS
63
muscular cells appear to be rich in glycogen. Since in the mesodermal
primitive plasm (the yellow crescent) there was no glycogen, one must
assume that the actual presence of glycogen in the muscular cells is due
to a new synthesis, probably in preparation for muscular function.
In fact the Ascidian tadpole, before entering metamorphosis, displays
a conspicuous muscular activity, which is probably supported by
glycogen. During the period of metamorphosis, very profound transformations take place in the larva: the intestine becomes highly functional,
and the tunic is secreted. It is probable that glycogen, in the intestine
and in the epidermis, may be utilized for the energy of metamorphosis
and the production of tunicine.
4. Glutathione
Particular attention has been recently given to the glutathione
content of the egg (Mazia and Dan, 1952). According to an early assumption of Rapkine (1931), which has been found fundamentally correct, a
cyclic reversible denaturation of some proteins takes place at every
mitosis. SH groups are thus formed which reduce the oxidized glutathione; on the other hand, because of their denaturation, the proteins
pass from the globular form to the fibrous one, and the spindle fibres
arise as a result of this oxidation (cf. Brächet, 1957, p. 212).
In the Ascidian egg the presence of glutathione was noticed by Ries
(1937). According to this author, the quantity was very small, and
perhaps was unequally distributed in the blastomeres of the segmenting
egg. The presence of glutathione in the Ascidian egg has been confirmed
by Urbani and Urbani-Mistruzzi (1947). Recently the problem has been
considered by Mancuso (1952a), who followed the distribution of
glutathione from the unsegmented egg up to the swimming larva.
According to this research, some glutathione appears to be localized in
the region of the mitotic spindles, where it probably carries out the
function which Rapkine attributed to it; but glutathione which is not
implicated with the spindles can also be found diffuse throughout the
cytoplasm of the vegetal blastomeres at the 8-cell stage. In more
advanced stages of development, the glutathione appears localized
mainly in the entodermal cells; in the swimming larva, it appears
localized only in the intestine. This intestinal localization of glutathione
would suggest that it might intervene in the processes connected with
the breakdown and subsequent utilization of the yolk proteins.
5. The Enzymes
To establish where the enzymes are segregated during development
is of importance.
(a) The first enzyme whose distribution was studied in the embryonic
63
muscular cells appear to be rich in glycogen. Since in the mesodermal
primitive plasm (the yellow crescent) there was no glycogen, one must
assume that the actual presence of glycogen in the muscular cells is due
to a new synthesis, probably in preparation for muscular function.
In fact the Ascidian tadpole, before entering metamorphosis, displays
a conspicuous muscular activity, which is probably supported by
glycogen. During the period of metamorphosis, very profound transformations take place in the larva: the intestine becomes highly functional,
and the tunic is secreted. It is probable that glycogen, in the intestine
and in the epidermis, may be utilized for the energy of metamorphosis
and the production of tunicine.
4. Glutathione
Particular attention has been recently given to the glutathione
content of the egg (Mazia and Dan, 1952). According to an early assumption of Rapkine (1931), which has been found fundamentally correct, a
cyclic reversible denaturation of some proteins takes place at every
mitosis. SH groups are thus formed which reduce the oxidized glutathione; on the other hand, because of their denaturation, the proteins
pass from the globular form to the fibrous one, and the spindle fibres
arise as a result of this oxidation (cf. Brächet, 1957, p. 212).
In the Ascidian egg the presence of glutathione was noticed by Ries
(1937). According to this author, the quantity was very small, and
perhaps was unequally distributed in the blastomeres of the segmenting
egg. The presence of glutathione in the Ascidian egg has been confirmed
by Urbani and Urbani-Mistruzzi (1947). Recently the problem has been
considered by Mancuso (1952a), who followed the distribution of
glutathione from the unsegmented egg up to the swimming larva.
According to this research, some glutathione appears to be localized in
the region of the mitotic spindles, where it probably carries out the
function which Rapkine attributed to it; but glutathione which is not
implicated with the spindles can also be found diffuse throughout the
cytoplasm of the vegetal blastomeres at the 8-cell stage. In more
advanced stages of development, the glutathione appears localized
mainly in the entodermal cells; in the swimming larva, it appears
localized only in the intestine. This intestinal localization of glutathione
would suggest that it might intervene in the processes connected with
the breakdown and subsequent utilization of the yolk proteins.
5. The Enzymes
To establish where the enzymes are segregated during development
is of importance.
(a) The first enzyme whose distribution was studied in the embryonic