248
RUTH BELLAIRS
intracellular yolk is a complex lipoprotein in all stages of degradation
and that the lipid drops within it are products of its degradation
(Thomas, 1938). There is little direct evidence to decide between these
two views but indirect evidence, which tends to support Thomas, is as
follows. In the extra-embryonic yolk most of the proteins are bound
with lipids (see Section II, B, 1), so that it seems unlikely that they
would not also be bound with lipids in the intracellular yolk. Furthermore, anyone who examines a piece of unfixed area opaca stained with
Nile blue sulphate or neutral red must be impressed by the immense
variety of the staining reactions of the different yolk inclusions; it
seems unlikely that this complex picture would be visible if Grodzinski's
idea of a uniform aqueous protein fluid were correct.
Several of the investigators who have studied the intracellular yolk
have taken advantage of the technique of tissue culture to follow the
course of digestion from stage to stage in individual yolk drops (Grodzinski, 1930, 1946; Thomas, 1938; Datkowna, 1949a, b). Their observations have not always tallied and their interpretations as to the course
of events have, therefore, not been unanimous. These investigators have
all used the area vitellina as their source of culture material but the
ages of the material at the time of explan ting were not identical. It is
even possible that some of the yolk was not intracellular but was trapped
extracellularly between strands of myelin-form-like material (see
Section V, A) although all the authors appear to have taken trouble to
describe only that yolk present in well-defined cells. Finally, it must be
borne in mind that the digestion of the yolk in tissue culture may not
necessarily be identical with that under normal conditions, for the cells
containing the yolk undergo changes themselves, gradually becoming
spindle-shaped fibroblasts (Thomas, 1938). Fortunately, however, the
available evidence is supplemented by the observations of other authors
who have simply examined area vitellina and yolk sac wall taken at
intervals from developing eggs (Konopacka, 1933 ; Romanini, 1960).
The first conclusion from these investigations is that the lipid component of the intracellular yolk drops becomes more conspicuous.
According to Grodzinski, this is because the small lipid drops present in
the undigested yolk fuse together to form large lipid drops. He suggested
that this is due to the destruction of the emulgator, which he believed
covers the individual lipid droplets ; he 'destroyed' it by iodine vapour,
ether, dioxan mercuric chloride, though presumably it is normally
destroyed by upases. Konopacka, and Thomas, however, considered
that these lipid drops were formed by the breakdown of the lipoproteins.
Konopacka also reported that some of the lipid formed in this way
became arranged as capsules around the phosphoprotein material.
There seems little doubt that the lipid itself undergoes a change at this
RUTH BELLAIRS
intracellular yolk is a complex lipoprotein in all stages of degradation
and that the lipid drops within it are products of its degradation
(Thomas, 1938). There is little direct evidence to decide between these
two views but indirect evidence, which tends to support Thomas, is as
follows. In the extra-embryonic yolk most of the proteins are bound
with lipids (see Section II, B, 1), so that it seems unlikely that they
would not also be bound with lipids in the intracellular yolk. Furthermore, anyone who examines a piece of unfixed area opaca stained with
Nile blue sulphate or neutral red must be impressed by the immense
variety of the staining reactions of the different yolk inclusions; it
seems unlikely that this complex picture would be visible if Grodzinski's
idea of a uniform aqueous protein fluid were correct.
Several of the investigators who have studied the intracellular yolk
have taken advantage of the technique of tissue culture to follow the
course of digestion from stage to stage in individual yolk drops (Grodzinski, 1930, 1946; Thomas, 1938; Datkowna, 1949a, b). Their observations have not always tallied and their interpretations as to the course
of events have, therefore, not been unanimous. These investigators have
all used the area vitellina as their source of culture material but the
ages of the material at the time of explan ting were not identical. It is
even possible that some of the yolk was not intracellular but was trapped
extracellularly between strands of myelin-form-like material (see
Section V, A) although all the authors appear to have taken trouble to
describe only that yolk present in well-defined cells. Finally, it must be
borne in mind that the digestion of the yolk in tissue culture may not
necessarily be identical with that under normal conditions, for the cells
containing the yolk undergo changes themselves, gradually becoming
spindle-shaped fibroblasts (Thomas, 1938). Fortunately, however, the
available evidence is supplemented by the observations of other authors
who have simply examined area vitellina and yolk sac wall taken at
intervals from developing eggs (Konopacka, 1933 ; Romanini, 1960).
The first conclusion from these investigations is that the lipid component of the intracellular yolk drops becomes more conspicuous.
According to Grodzinski, this is because the small lipid drops present in
the undigested yolk fuse together to form large lipid drops. He suggested
that this is due to the destruction of the emulgator, which he believed
covers the individual lipid droplets ; he 'destroyed' it by iodine vapour,
ether, dioxan mercuric chloride, though presumably it is normally
destroyed by upases. Konopacka, and Thomas, however, considered
that these lipid drops were formed by the breakdown of the lipoproteins.
Konopacka also reported that some of the lipid formed in this way
became arranged as capsules around the phosphoprotein material.
There seems little doubt that the lipid itself undergoes a change at this