172
SAUL WISCHNITZER
Beams and Kessel (1963) have found that the endoplasmic reticulum was
responsible for yolk formation in the crayfish. More recently, Roth and
Porter have (1964) reported that yolk accumulates by micropinocytosis
from the plasmalemma in the oocyte of the mosquito A edes aegypti. Sources
comparable to those just cited would, according to Wartenberg (1962),
appear to be simultaneously active in amphibians.
The wide variation in the method of formation of yolk platelets may
possibly be due to differences in the state of the raw materials present in
the oocyte. These raw materials are derived from the nutrient substances
that have been taken up from the surrounding environment. Within the
oocyte itself, synthesis takes place to form the end products. In cases where
the raw materials are amino acids, a complex mechanism such as the
endoplasmic reticulum is needed as a site for synthesis, sequestration, and
storage. On the other hand, where the raw materials perhaps are in a more
complex state, such as polypeptides, synthesis would be limited, and the
major activity would be that of sequestration and storage. In such cases,
these processes could be carried out solely within mitochondria. In view
of the well-known capability of mitochondria for pleomorphism (see, for
example, Kessel, 1963) the different manifestations of yolk platelet formation by these organelles is not surprising.
Until very recently, little was known about the mode of utilization of
the yolk platelets during embryogenesis. An insight into the structural
changes that the platelets undergo was derived from an electron-microscope
study by Sung (1962), who noted that in some cases the crystalline body
appears to cleave into parallel or concentric lamellae. More recently Karasaki (1963b) showed that the first change in the yolk platelets takes place
in certain embryonic areas as a diminution in the thickness (and later
disappearance) of the superficial layer. It is thought to be possible that this
layer contains polysaccharides and serves as a protective coat around the
crystalline core. With the removal of this layer, the core begins to decompose into fine particles. Newly formed vesicles, lamellae, and fibrils are
intimately associated with the decomposing platelets. The formation of
these structures is thought to be an intermediate step in the differentiation
of the essential cytomembranes. The cytomembranes, therefore, can be
looked upon as derivatives of yolk platelet material.
The papers by Saguchi (1932) and Wittek (1952) suggested that lipochondria arise toward the end of the previtellogenic period. This is in
agreement with the findings reported earlier in this paper.
Raven (1961) has reported that in amphibians the first fat bodies are
located in a juxtanuclear position and that, as with the mitochondria, they
then form a zone of lipoid bodies around the nucleus. The elements ultimately become more or less evenly distributed throughout the cytoplasm.
SAUL WISCHNITZER
Beams and Kessel (1963) have found that the endoplasmic reticulum was
responsible for yolk formation in the crayfish. More recently, Roth and
Porter have (1964) reported that yolk accumulates by micropinocytosis
from the plasmalemma in the oocyte of the mosquito A edes aegypti. Sources
comparable to those just cited would, according to Wartenberg (1962),
appear to be simultaneously active in amphibians.
The wide variation in the method of formation of yolk platelets may
possibly be due to differences in the state of the raw materials present in
the oocyte. These raw materials are derived from the nutrient substances
that have been taken up from the surrounding environment. Within the
oocyte itself, synthesis takes place to form the end products. In cases where
the raw materials are amino acids, a complex mechanism such as the
endoplasmic reticulum is needed as a site for synthesis, sequestration, and
storage. On the other hand, where the raw materials perhaps are in a more
complex state, such as polypeptides, synthesis would be limited, and the
major activity would be that of sequestration and storage. In such cases,
these processes could be carried out solely within mitochondria. In view
of the well-known capability of mitochondria for pleomorphism (see, for
example, Kessel, 1963) the different manifestations of yolk platelet formation by these organelles is not surprising.
Until very recently, little was known about the mode of utilization of
the yolk platelets during embryogenesis. An insight into the structural
changes that the platelets undergo was derived from an electron-microscope
study by Sung (1962), who noted that in some cases the crystalline body
appears to cleave into parallel or concentric lamellae. More recently Karasaki (1963b) showed that the first change in the yolk platelets takes place
in certain embryonic areas as a diminution in the thickness (and later
disappearance) of the superficial layer. It is thought to be possible that this
layer contains polysaccharides and serves as a protective coat around the
crystalline core. With the removal of this layer, the core begins to decompose into fine particles. Newly formed vesicles, lamellae, and fibrils are
intimately associated with the decomposing platelets. The formation of
these structures is thought to be an intermediate step in the differentiation
of the essential cytomembranes. The cytomembranes, therefore, can be
looked upon as derivatives of yolk platelet material.
The papers by Saguchi (1932) and Wittek (1952) suggested that lipochondria arise toward the end of the previtellogenic period. This is in
agreement with the findings reported earlier in this paper.
Raven (1961) has reported that in amphibians the first fat bodies are
located in a juxtanuclear position and that, as with the mitochondria, they
then form a zone of lipoid bodies around the nucleus. The elements ultimately become more or less evenly distributed throughout the cytoplasm.