230
RUTH BELLAIRS
Radomski et ah, 1963) and of phosvitin (Hasegawa, 1960b; Burley and
Cook, 1961; McCully et ah, 1962; Radomski et ah, 1963). The different
authors did not find the same proportions of these substances in the
'granules'. However, it appears that 60-70% consists of lipovitellins,
whereas between 16 and 26% consists of phosvitin.
During ultracentrifugation the yolk spheres sediment. I t should
thus be possible to examine their chemical content, though this does
not yet appear to have been done. The result obtained is, however,
likely to differ according to the method of preparation, for some investigators routinely dilute samples of yolk with water or saline before centrifugation, whereas others do not. In a trial run (8 h at 20,000 rev/min
in a Spinco, Model L, using type 40 head at room temperature) I found
that in the diluted samples the yolk spheres had burst, whereas in the
undiluted yolk they had remained intact even though slightly distorted.
Although they do not necessarily break up on centrifugation they do
so during incubation and this is apparently brought about by some disturbance of the yolk sphere surface. According to Grodzinski (1939,
1947) there is a substantial membrane around each yolk sphere which he
regards as being a physiologically semipermeable membrane. His
evidence is based on studies aimed at altering the osmotic pressure in
the continuous phase fluid. For instance, he found that if he increased
the osmotic pressure by adding a crystal of sodium chloride the yolk
spheres became smaller (Grodzinski, 1946). Similar results were obtained by Swiezawska (1949). Conversely, if more water was added, the
yolk spheres burst (Grodzinski, 1951). Such evidence does not, however,
provide a completely convincing case that a membrane is present, for
gels may also shrink and swell in this way (Alexander and Johnson,
1949).
Unfortunately, an examination by electron microscopy of the yolk
spheres did not completely resolve the problem (Bellairs, 1961b). Three
different conditions were found.
(a) The naked surface. Generally it was found that the surface was
completely naked, with certain exceptions (see below), and for various
reasons (see Bellairs, 1961b) it was decided that this was not a fixation
artifact, nor was it an ageing phenomenon in the egg. It was concluded,
therefore, that very few of the yolk spheres were surrounded by a
membrane and this was supported by a polarized light study.
(b) The lamellated capsule. A small proportion of yolk spheres did have
a membrane-like capsule around them though it seldom resembled a
cell membrane. Typically, it was a lamellated capsule though the
number of layers was not constant. The periodicity of the individual
bands was about 40 Â (i.e. the full width of one dark and one light band),
which resembled that of myelin figures. It is of especial interest, there-
RUTH BELLAIRS
Radomski et ah, 1963) and of phosvitin (Hasegawa, 1960b; Burley and
Cook, 1961; McCully et ah, 1962; Radomski et ah, 1963). The different
authors did not find the same proportions of these substances in the
'granules'. However, it appears that 60-70% consists of lipovitellins,
whereas between 16 and 26% consists of phosvitin.
During ultracentrifugation the yolk spheres sediment. I t should
thus be possible to examine their chemical content, though this does
not yet appear to have been done. The result obtained is, however,
likely to differ according to the method of preparation, for some investigators routinely dilute samples of yolk with water or saline before centrifugation, whereas others do not. In a trial run (8 h at 20,000 rev/min
in a Spinco, Model L, using type 40 head at room temperature) I found
that in the diluted samples the yolk spheres had burst, whereas in the
undiluted yolk they had remained intact even though slightly distorted.
Although they do not necessarily break up on centrifugation they do
so during incubation and this is apparently brought about by some disturbance of the yolk sphere surface. According to Grodzinski (1939,
1947) there is a substantial membrane around each yolk sphere which he
regards as being a physiologically semipermeable membrane. His
evidence is based on studies aimed at altering the osmotic pressure in
the continuous phase fluid. For instance, he found that if he increased
the osmotic pressure by adding a crystal of sodium chloride the yolk
spheres became smaller (Grodzinski, 1946). Similar results were obtained by Swiezawska (1949). Conversely, if more water was added, the
yolk spheres burst (Grodzinski, 1951). Such evidence does not, however,
provide a completely convincing case that a membrane is present, for
gels may also shrink and swell in this way (Alexander and Johnson,
1949).
Unfortunately, an examination by electron microscopy of the yolk
spheres did not completely resolve the problem (Bellairs, 1961b). Three
different conditions were found.
(a) The naked surface. Generally it was found that the surface was
completely naked, with certain exceptions (see below), and for various
reasons (see Bellairs, 1961b) it was decided that this was not a fixation
artifact, nor was it an ageing phenomenon in the egg. It was concluded,
therefore, that very few of the yolk spheres were surrounded by a
membrane and this was supported by a polarized light study.
(b) The lamellated capsule. A small proportion of yolk spheres did have
a membrane-like capsule around them though it seldom resembled a
cell membrane. Typically, it was a lamellated capsule though the
number of layers was not constant. The periodicity of the individual
bands was about 40 Â (i.e. the full width of one dark and one light band),
which resembled that of myelin figures. It is of especial interest, there-