218
RUTH BELLAIRS
within the last two decades that some of the most exciting discoveries
have been made.
The story of yolk must start with its formation. The materials from
which it is composed are manufactured in the liver of the laying hen and
transported in the blood to that remarkable cell, the oocyte, where they
become physically rearranged in relationship to one another. Everything that enters the oocyte must pass through the follicle, and so the
morphological and physiological relationships between follicle and
oocyte are of particular interest to us. The size of the yolk is so vast that
a special membrane, the vitelline membrane, is made to support it.
It is well known that if an egg possesses large amounts of yolk it needs
specialized modes of cleavage and gastrulation. It is perhaps less obvious,
though equally important, that the presence of a large yolk has repercussions throughout the entire embryonic life. For instance, enzymes
are needed to digest the yolk, and the product must be transported to
the developing tissues.
Although this chapter is concerned primarily with yolk, much of the
work reported here has a wider biological significance. For example,
the mechanism by which yolk is ingested is of relevance to any discussion on the intake of large particles by living cells.
The story of yolk is not yet complete, but one fact is fundamental to
our understanding. This is, that although it has been suggested many
times that extracellular yolk may become directly converted into cells,
there is no convincing evidence that this is so. The yolk is simply a
source of raw materials and it can only contribute to protoplasm if it is
ingested by living cells and subjected to enzyme activity.
In this chapter I have attempted to summarize some of the knowledge
that bears on these problems. Many of the points I have raised were
considered in the classical works of Needham (1931, 1950) and Romanoff
and Romanoff (1949). It has not been possible to discuss all the newer
work here, but further information will be found in the reviews of
Williams (1964).
II. The Laid Egg
The yolk comprises about 30% by weight of the laid egg. The remainder consists of the egg-white, shell and shell membranes (see Fig.
1). For a description of these other components, see Romanoff and
Romanoff (1949).
The yolk is a solution and so is the egg-white, and they are kept apart
by the vitelline membrane. Therefore, in order to understand something about the physiology of the laid egg and the developing embryo
it is important to be familiar with the structure of the vitelline membrane.
RUTH BELLAIRS
within the last two decades that some of the most exciting discoveries
have been made.
The story of yolk must start with its formation. The materials from
which it is composed are manufactured in the liver of the laying hen and
transported in the blood to that remarkable cell, the oocyte, where they
become physically rearranged in relationship to one another. Everything that enters the oocyte must pass through the follicle, and so the
morphological and physiological relationships between follicle and
oocyte are of particular interest to us. The size of the yolk is so vast that
a special membrane, the vitelline membrane, is made to support it.
It is well known that if an egg possesses large amounts of yolk it needs
specialized modes of cleavage and gastrulation. It is perhaps less obvious,
though equally important, that the presence of a large yolk has repercussions throughout the entire embryonic life. For instance, enzymes
are needed to digest the yolk, and the product must be transported to
the developing tissues.
Although this chapter is concerned primarily with yolk, much of the
work reported here has a wider biological significance. For example,
the mechanism by which yolk is ingested is of relevance to any discussion on the intake of large particles by living cells.
The story of yolk is not yet complete, but one fact is fundamental to
our understanding. This is, that although it has been suggested many
times that extracellular yolk may become directly converted into cells,
there is no convincing evidence that this is so. The yolk is simply a
source of raw materials and it can only contribute to protoplasm if it is
ingested by living cells and subjected to enzyme activity.
In this chapter I have attempted to summarize some of the knowledge
that bears on these problems. Many of the points I have raised were
considered in the classical works of Needham (1931, 1950) and Romanoff
and Romanoff (1949). It has not been possible to discuss all the newer
work here, but further information will be found in the reviews of
Williams (1964).
II. The Laid Egg
The yolk comprises about 30% by weight of the laid egg. The remainder consists of the egg-white, shell and shell membranes (see Fig.
1). For a description of these other components, see Romanoff and
Romanoff (1949).
The yolk is a solution and so is the egg-white, and they are kept apart
by the vitelline membrane. Therefore, in order to understand something about the physiology of the laid egg and the developing embryo
it is important to be familiar with the structure of the vitelline membrane.