ULTRASTRUCTURE OF DEVELOPING AMPHIBIAN EGG 171
cytoplasm of immature oocytes from Triton contained peculiar previtelline
bodies (complex cytosomes). Each of these contains microgranules, fine
vesicles, and lamellar structures, all of which are surrounded by their own
membrane. This structure is considered to represent a transition stage
produced by the contribution of material from multiple sources, including
the cytoplasmic organelles (especially the mitochondria; Fig. 24) and
nucleus. In the center of this body the material becomes organized and
thickened to form the crystalline core.
The most recent investigation of the yolk formation process has been
made by Balinsky and Devis (1963) with Xenopus oocytes. They reported
that most yolk is formed from special peripherally located mitochondria
having poorly developed cristae (Fig. 24). These mitochondria become
modified into precursor granules by undergoing a change in shape and
internal organization. Thus, they are round or oval rather than elongated,
and their internal membranes are concentrically arranged. As the oocytes
mature, there is an increase in these concentric membranes, and they
become evenly spaced, usually parallel to the surface of the granules. By
fusion, the membranes are presumed to form the crystalline core of the
platelet.
Balinsky and Devis also report that during later growth stages, yolk
can be formed from multivesicular bodies. They think that these structures
undergo transformation into either yolk platelets or pigment granules (see
p. 175). In the former case, the multivesicular bodies become filled with fine
granules and somehow change into yolk platelets.
It is apparent from the previous considerations of possible methods of
yolk platelet formation that, whereas some gains in our understanding of
the process of yolk formation have been made, the problem is still unresolved. It seems certain that mitochondria are directly involved in the
formation of platelets; however, this appears limited to anurans (see p.
169). It is conceivable that not only is there a difference between the two
amphibian groups, but that within the same group different modes of yolk
platelet formation exist.
The participation of the multivesicular bodies in yolk formation is not
so well established and may also be limited to anurans. In urodeles, such
as Triturus, the method of yolk formation remains very uncertain. Its
formation from complex cytosomes, as has been suggested by Wartenberg
(1962), is an attractive idea, but remains to be confirmed by studies in
different urodele forms. This concept is the most inclusive since it encompasses many contributary sources for yolk formation. However, in nonamphibian forms, only one intracytoplasmic source has been assigned the
responsibility for vitellogenesis. In this connection Favard and Carasso
(1958) have implicated the mitochondria in the mollusk Planorbis, whereas
cytoplasm of immature oocytes from Triton contained peculiar previtelline
bodies (complex cytosomes). Each of these contains microgranules, fine
vesicles, and lamellar structures, all of which are surrounded by their own
membrane. This structure is considered to represent a transition stage
produced by the contribution of material from multiple sources, including
the cytoplasmic organelles (especially the mitochondria; Fig. 24) and
nucleus. In the center of this body the material becomes organized and
thickened to form the crystalline core.
The most recent investigation of the yolk formation process has been
made by Balinsky and Devis (1963) with Xenopus oocytes. They reported
that most yolk is formed from special peripherally located mitochondria
having poorly developed cristae (Fig. 24). These mitochondria become
modified into precursor granules by undergoing a change in shape and
internal organization. Thus, they are round or oval rather than elongated,
and their internal membranes are concentrically arranged. As the oocytes
mature, there is an increase in these concentric membranes, and they
become evenly spaced, usually parallel to the surface of the granules. By
fusion, the membranes are presumed to form the crystalline core of the
platelet.
Balinsky and Devis also report that during later growth stages, yolk
can be formed from multivesicular bodies. They think that these structures
undergo transformation into either yolk platelets or pigment granules (see
p. 175). In the former case, the multivesicular bodies become filled with fine
granules and somehow change into yolk platelets.
It is apparent from the previous considerations of possible methods of
yolk platelet formation that, whereas some gains in our understanding of
the process of yolk formation have been made, the problem is still unresolved. It seems certain that mitochondria are directly involved in the
formation of platelets; however, this appears limited to anurans (see p.
169). It is conceivable that not only is there a difference between the two
amphibian groups, but that within the same group different modes of yolk
platelet formation exist.
The participation of the multivesicular bodies in yolk formation is not
so well established and may also be limited to anurans. In urodeles, such
as Triturus, the method of yolk formation remains very uncertain. Its
formation from complex cytosomes, as has been suggested by Wartenberg
(1962), is an attractive idea, but remains to be confirmed by studies in
different urodele forms. This concept is the most inclusive since it encompasses many contributary sources for yolk formation. However, in nonamphibian forms, only one intracytoplasmic source has been assigned the
responsibility for vitellogenesis. In this connection Favard and Carasso
(1958) have implicated the mitochondria in the mollusk Planorbis, whereas