ULTRASTRUCTURE OF DEVELOPING AMPHIBIAN EGG 175
why many theories have suggested different sites. In reality, all the theories
would have some measure of validity since they are not mutually exclusive.
Balinsky and Devis (1963) claimed to have demonstrated the actual
pigment precursors which, according to these investigators, are free of
melanin or differentiated granules. They believe that each of the precursors
is, in actuality, a vesicle that encloses a group of smaller such subunits, the
multivesicular bodies (see p. 171). The latter can give rise to pigment
granules by a gradual conversion of the multivesicular body into one containing coarse granules. Such granules tend to form larger clumps at the
periphery. Subsequently the precursor may, but need not necessarily, be
transformed into a crystalline lattice in which the spacing is wider than in
the core of the yolk platelets. Starting at their peripheries, melanin is
deposited in these bodies during this entire process as the precursor gradually increases in size. Thus, the multivesicular bodies average 0.17 μ in
diameter, the coarse granular bodies 0.24 μ, and the mature pigment
granules 0.46 μ.
In conclusion, it is clear that although the ultrastructure of the pigment
granule has been elucidated, its exact method of formation is still uncertain.
The problem may, perhaps, be clarified by studying a series of oocytes in
which the different growth stages are separated by small intervals.
IV. Summary
This review has attempted to correlate and evaluate the results obtained
from the numerous electron-microscope studies of both the amphibian
egg and its surrounding layers.
1. It has been established that three cellular layers and an inner acellular
zona pellucida surround the maturing egg. The cellular layers result from
the organization of existing cellular elements around the enlarging oocyte.
The formation of the zona pellucida involves two simultaneously occurring
phenomena. The first consists of the deposition of a homogeneous granular
material between the basal membrane of the follicle cell and the plasmalemma of the ovum. The second phenomenon involves the downward
extension as macrovilli of the basal follicle cell membrane and upward
projection as microvilli of the plasmalemma surrounding the oocyte. The
functional significance of the zona pellucida is discussed.
2. The growth of the oocyte has been reviewed, and the ultrastructural
changes taking place within it have been analyzed. The mitochondria of the
amphibian oocyte have the same ultrastructure as those of somatic cells.
In some forms, a yolk nucleus which consists of a cluster of mitochondria
becomes organized adjacent to the nucleus. Nucleolar material may be
why many theories have suggested different sites. In reality, all the theories
would have some measure of validity since they are not mutually exclusive.
Balinsky and Devis (1963) claimed to have demonstrated the actual
pigment precursors which, according to these investigators, are free of
melanin or differentiated granules. They believe that each of the precursors
is, in actuality, a vesicle that encloses a group of smaller such subunits, the
multivesicular bodies (see p. 171). The latter can give rise to pigment
granules by a gradual conversion of the multivesicular body into one containing coarse granules. Such granules tend to form larger clumps at the
periphery. Subsequently the precursor may, but need not necessarily, be
transformed into a crystalline lattice in which the spacing is wider than in
the core of the yolk platelets. Starting at their peripheries, melanin is
deposited in these bodies during this entire process as the precursor gradually increases in size. Thus, the multivesicular bodies average 0.17 μ in
diameter, the coarse granular bodies 0.24 μ, and the mature pigment
granules 0.46 μ.
In conclusion, it is clear that although the ultrastructure of the pigment
granule has been elucidated, its exact method of formation is still uncertain.
The problem may, perhaps, be clarified by studying a series of oocytes in
which the different growth stages are separated by small intervals.
IV. Summary
This review has attempted to correlate and evaluate the results obtained
from the numerous electron-microscope studies of both the amphibian
egg and its surrounding layers.
1. It has been established that three cellular layers and an inner acellular
zona pellucida surround the maturing egg. The cellular layers result from
the organization of existing cellular elements around the enlarging oocyte.
The formation of the zona pellucida involves two simultaneously occurring
phenomena. The first consists of the deposition of a homogeneous granular
material between the basal membrane of the follicle cell and the plasmalemma of the ovum. The second phenomenon involves the downward
extension as macrovilli of the basal follicle cell membrane and upward
projection as microvilli of the plasmalemma surrounding the oocyte. The
functional significance of the zona pellucida is discussed.
2. The growth of the oocyte has been reviewed, and the ultrastructural
changes taking place within it have been analyzed. The mitochondria of the
amphibian oocyte have the same ultrastructure as those of somatic cells.
In some forms, a yolk nucleus which consists of a cluster of mitochondria
becomes organized adjacent to the nucleus. Nucleolar material may be