244
R U T H B E L L A I R S
into the large mass of extra-embryonic yolk so that their activity is
extracellular. Unfortunately, however, many investigators have not
distinguished between the yolk and the yolk sac when making chemical
analyses so that it is not always possible to be sure that the enzymes
produced by the yolk sac act intracellularly. According to Romanoff
(1960), during the early stages yolk is phagocytosed and acted upon
intracellularly. During the later stages phagocytosis ceases and the
enzymes are passed out into the yolk sac and act intracellularly.
During the early stages of development, intracellular yolk is present
in all types of cells throughout the blastoderm. Those in the area
pellucida are not easy to see by light microscopy and have frequently
been overlooked. They are, however, clearly visible by electron microscopy (Bellairs, 1958; Balinsky and Walther, 1961). From our knowledge of the extracellular yolk it is considered that the intracellular
yolk also consists mainly of proteins and lipids. It is probable, however,
that they are not combined into exactly the same compounds as in the
extracellular yolk (see below).
When the intracellular yolk drops are examined by light microscopy,
their boundaries are sharply defined (Thomas, 1938; Grodzinski, 1946),
and Grodzinski concluded that this was because each was surrounded
by a membrane. This was confirmed when the material was examined
by electron microscopy and a unit membrane (see Section II, C) was
seen to surround each yolk drop. On the basis of staining reactions,
Grodzinski concluded further that the yolk drops observed within the
cells were identical with the extracellular yolk spheres. However, this
now seems less likely since it has been shown by electron microscopy
that a unit membrane is normally not present around each extracellular
yolk sphere.
An intracellular yolk drop can probably originate in either of two
ways. It may, for instance, be the original yolk sphere of the oocyte
which has been constantly surrounded by cytoplasm and has, therefore,
retained its own unit membrane (see Section III, A, 3). It seems probable
that the yolk drops of the area pellucida fall into this category. Other
yolk drops, however (i.e. many of those of the area opaca), have probably formed by phagocytosis of the underlying yolk (see Section V, D).
The unit membrane of each of these yolk drops is then presumably
formed by an invagination and pinching off of a part of the cell membrane. This possibility is supported indirectly by the fact that in both
instances where it seems likely that structures have been phagocytosed
(i.e. colloidal gold (Bellairs and New, 1962), or lumps of degenerating
cells (Bellairs, 1961a)), the phagocytosed material is surrounded by a
unit membrane.
Datkowna (1949a) has studied the effect of hypotonie solutions on
R U T H B E L L A I R S
into the large mass of extra-embryonic yolk so that their activity is
extracellular. Unfortunately, however, many investigators have not
distinguished between the yolk and the yolk sac when making chemical
analyses so that it is not always possible to be sure that the enzymes
produced by the yolk sac act intracellularly. According to Romanoff
(1960), during the early stages yolk is phagocytosed and acted upon
intracellularly. During the later stages phagocytosis ceases and the
enzymes are passed out into the yolk sac and act intracellularly.
During the early stages of development, intracellular yolk is present
in all types of cells throughout the blastoderm. Those in the area
pellucida are not easy to see by light microscopy and have frequently
been overlooked. They are, however, clearly visible by electron microscopy (Bellairs, 1958; Balinsky and Walther, 1961). From our knowledge of the extracellular yolk it is considered that the intracellular
yolk also consists mainly of proteins and lipids. It is probable, however,
that they are not combined into exactly the same compounds as in the
extracellular yolk (see below).
When the intracellular yolk drops are examined by light microscopy,
their boundaries are sharply defined (Thomas, 1938; Grodzinski, 1946),
and Grodzinski concluded that this was because each was surrounded
by a membrane. This was confirmed when the material was examined
by electron microscopy and a unit membrane (see Section II, C) was
seen to surround each yolk drop. On the basis of staining reactions,
Grodzinski concluded further that the yolk drops observed within the
cells were identical with the extracellular yolk spheres. However, this
now seems less likely since it has been shown by electron microscopy
that a unit membrane is normally not present around each extracellular
yolk sphere.
An intracellular yolk drop can probably originate in either of two
ways. It may, for instance, be the original yolk sphere of the oocyte
which has been constantly surrounded by cytoplasm and has, therefore,
retained its own unit membrane (see Section III, A, 3). It seems probable
that the yolk drops of the area pellucida fall into this category. Other
yolk drops, however (i.e. many of those of the area opaca), have probably formed by phagocytosis of the underlying yolk (see Section V, D).
The unit membrane of each of these yolk drops is then presumably
formed by an invagination and pinching off of a part of the cell membrane. This possibility is supported indirectly by the fact that in both
instances where it seems likely that structures have been phagocytosed
(i.e. colloidal gold (Bellairs and New, 1962), or lumps of degenerating
cells (Bellairs, 1961a)), the phagocytosed material is surrounded by a
unit membrane.
Datkowna (1949a) has studied the effect of hypotonie solutions on