174
SAUL WISCHNITZER
absence of cortical granules delays the process of liquefaction that would
otherwise take place inside the perivitelline space, and, thus, rotation is
possible only in the region outside of the fertilization membrane.
Amphibian oocytes contain pigment granules having an ultrastructure
that is demonstrably different in the two orders Urodela and Anura. While
the ultrastructure has, to a large extent, been clarified, the mechanisms
underlying pigment granule formation still await resolution. In this connection, numerous theories have been proposed over the last half century.
Weissenf els (1956) has indicated that all these theories concern four possible
sources: mitochondria, the Golgi complex, lipochondria, and nucleus. The
more current theories ascribe the formation of pigment granules to only
the first two sources (Horstmann, 1957). All recent electron-microscope
studies indicate that the pigment precursors develop in vesicles that are
associated with the Golgi complex and the endoplasmic reticulum.
In the amphibian egg, the formation of pigment inside of vesicles was
originally noted by Wartenberg and Gusek (1959). The origin of the vesicles
are ascribed to the endoplasmic reticulum. More recently, Wartenberg
(1962) has described the process of pigment formation in greater detail.
Thus, in Xenopus, he noted that young oocytes possess vesicles, 0.01-0.02 μ
in diameter, which contain microgranules 100-200 A in diameter. Gradually
these vesicles become darker, and they develop into large solid bodies, either
by the synthesis of more material or by fusion of existing material. These
membrane-enclosed pigment precursor bodies will enlarge up to 0.5 μ in
diameter. In Triturus oocytes, which contain many vesicles, the precursors
are recognized by the presence within them of dense microgranules. These
vesicles increase in size, but their microgranules remain distinct.
The origin of the vesicles is also associated with unsolved problems.
Wartenberg (1962) has withdrawn his idea that they are derivatives of the
endoplasmic reticulum, since such an organized membrane system does not
exist in the developing amphibian oocyte. Rather, he suggests that pigment
formation is a function of what he calls the "vacuolar system" of the cell.
By this system he means that various intracellular sources contribute to
the presence of the numerous cytoplasmic vesicles, such as the Golgi complex, mitochondria, and the nucleus. The contribution of each of these
elements to the supply of vesicles presumably is dependent upon physiological conditions and cell type. In any case, the sources of vesicles would
be only indirectly involved in pigment formation. Thus, Wartenberg
suggests that in Xenopus, for example, the vesicles are capable of taking up
cytoplasmic material, presumably proteins, which is transformed into the
various structures and crystalloid formations. These structures then become
embedded in the pigment matrix which contains melanin.
The multisource theory of pigment formation, Wartenberg feels, explains
SAUL WISCHNITZER
absence of cortical granules delays the process of liquefaction that would
otherwise take place inside the perivitelline space, and, thus, rotation is
possible only in the region outside of the fertilization membrane.
Amphibian oocytes contain pigment granules having an ultrastructure
that is demonstrably different in the two orders Urodela and Anura. While
the ultrastructure has, to a large extent, been clarified, the mechanisms
underlying pigment granule formation still await resolution. In this connection, numerous theories have been proposed over the last half century.
Weissenf els (1956) has indicated that all these theories concern four possible
sources: mitochondria, the Golgi complex, lipochondria, and nucleus. The
more current theories ascribe the formation of pigment granules to only
the first two sources (Horstmann, 1957). All recent electron-microscope
studies indicate that the pigment precursors develop in vesicles that are
associated with the Golgi complex and the endoplasmic reticulum.
In the amphibian egg, the formation of pigment inside of vesicles was
originally noted by Wartenberg and Gusek (1959). The origin of the vesicles
are ascribed to the endoplasmic reticulum. More recently, Wartenberg
(1962) has described the process of pigment formation in greater detail.
Thus, in Xenopus, he noted that young oocytes possess vesicles, 0.01-0.02 μ
in diameter, which contain microgranules 100-200 A in diameter. Gradually
these vesicles become darker, and they develop into large solid bodies, either
by the synthesis of more material or by fusion of existing material. These
membrane-enclosed pigment precursor bodies will enlarge up to 0.5 μ in
diameter. In Triturus oocytes, which contain many vesicles, the precursors
are recognized by the presence within them of dense microgranules. These
vesicles increase in size, but their microgranules remain distinct.
The origin of the vesicles is also associated with unsolved problems.
Wartenberg (1962) has withdrawn his idea that they are derivatives of the
endoplasmic reticulum, since such an organized membrane system does not
exist in the developing amphibian oocyte. Rather, he suggests that pigment
formation is a function of what he calls the "vacuolar system" of the cell.
By this system he means that various intracellular sources contribute to
the presence of the numerous cytoplasmic vesicles, such as the Golgi complex, mitochondria, and the nucleus. The contribution of each of these
elements to the supply of vesicles presumably is dependent upon physiological conditions and cell type. In any case, the sources of vesicles would
be only indirectly involved in pigment formation. Thus, Wartenberg
suggests that in Xenopus, for example, the vesicles are capable of taking up
cytoplasmic material, presumably proteins, which is transformed into the
various structures and crystalloid formations. These structures then become
embedded in the pigment matrix which contains melanin.
The multisource theory of pigment formation, Wartenberg feels, explains