Meaning of Extraembryonic Structures in Chick Embryo:
The embryo of chick possesses four extraembryonic or foetal membranes: namely, the yolk sac, the allantois, the amnion and the serosa or chorion. In amphibian embryo, the yolk sac and the allantois are present in rudimentary condition.
The amnion and the serosa are developed in the reptilian embryo for the first time in evolutionary history of the vertebrates. In birds these two structures are retained while in mammals these are also present in a modified form. All the extraembryonic membranes are discarded at hatching while the yolk sac is incorporated into the small intestine.
As development goes on, the closely set ectoderm and somatopleure (somatic mesoderm) as well as the endoderm and splanchnopleure (splanchnic mesoderm) extend into the extraembryonic area. The developing embryo becomes located at the central area of the blastodisc.
The embryo becomes separated by undercutting grooves formed by limiting body folds. These folds initiate with the formation of crescentic head fold which extends backward as the body folds. The body folds subsequently merge with the tail fold. By this, way the embryo undercuts and separates itself from the underlying yolk mass.
The yolk, though supplies nourishment to the embryo, is not considered as part of the embryo. But in cases where the quantity of yolk is very large, special yolk sac develops to store the yolk. As the embryo raises itself, the yolk is enclosed by the yolk sac. The yolk sac develops from the margin of the blastoderm which advances around the yolk mass until it surrounds the mass completely.
In chick embryo, the gut is represented by a round cavity beneath the primitive streak at about sixteen hours of incubation. At about 24 hours, a small enteric pocket develops into the developing head as the foregut. The foregut is surrounded by mesoderm.
At about 48-hour stage, with the formation of tail bud, a sac-like hind- gut develops. The undifferentiated portion of the gut between the fore- and hindgut is called the midgut which remains as a small portion of the small intestine as the embryo grows older.
The midgut leads into the reduced yolk sac by yolk stalk (Fig. 5.43 A1). The yolk sac becomes smaller due to consumption of the yolk and before hatching the yolk sac exists as a projection from the small intestine. The yolk sac is covered by splanchnopleure. The yolk sac is finally incorporated with the small intestine.
The yolk is used as food by the embryo. The endoderm of the yolk sac secretes enzymes which break the yolk into diffusible substances. These substances are carried by the vitelline veins and finally to the heart. From the heart these are conveyed to the different parts of the embryo and extraembryonic structures.
The amnion and the serosa (or chorion) are the two extraembryonic membranes which develop together. The amnion is actually a membrane covering the embryo and thus housing it like a bag. It separates the embryo from the immediate contact with the environment. Between the embryo and the amnion, there is a space called the amniotic cavity containing amniotic fluid, a saline solution.
The amnion as well as the amniotic fluid protect the developing embryo from desiccation and also equalise the pressure against
the embryo by physical forces. The fluid acts as a buffer and gives protection from any kind of shock. Adhesion of the embryo to the amniotic wall is prevented by the rhythmic movement of the amnion.
As the embryo grows, the weight of the body is enhanced and as a result the embryo-sinks into the soft yolk. Due to this, the blastodisc remains elevated over the embryo. At 30 hours of incubation, a well- formed head of the embryo of chick pushes forward and sinks into the underlying yolk.
This causes the elevation of a blastodermic fold called the amniotic head fold. This fold bends backward over the head and covers it as a hood.
The lateral ends of the head fold prolong backward along the two sides of the embryo as lateral amniotic folds. The lateral folds come over the embryo and fuse with each other along the anteroposterior direction. As a consequence of fusion, the embryo becomes more or less covered by folds.
After 48 hours of incubation another fold appears from the posterior end of the embryo (tail bud) which is called the amniotic tail fold. This fold proceeds forward over the embryo and finally meets and fuses with amniotic head fold during the 4th day of incubation (Fig. 5.43 A2). The region of union of the amniotic folds is marked by a scar called the seroamniotic connection.
The amniotic fold is composed of two layers—an inner and an outer. So the fusion of such folds will result in the formation of an inner and an outer layer. The inner layer is named as the amnion and the outer one is called the serosa (the name chorion is reserved from the same layer in mammals by many embryologists). The amnion is composed of somatic mesoderm on the outer side and ectoderm on the inner side.
The serosa has the somatic mesoderm on the inner side and the ectoderm on the outerside (Fig. 5.43 A2). In between the serosa and the amnion lies the extraembryonic coelom which is continuous with the intraembryonic coelom of the embryo proper. The serosa grows around the yolk sac and encloses it completely at the end of second week.
Formation of Allantois:
The allantois develops in conjunction with the serosa and differs in nature from the other extraembryonic membranes. It is not discarded in mammals but its intraembryonic portion is retained as the urinary bladder.
The allantois starts its development as a ventral diverticulum in the 72-hour old chick embryo from the hindgut. This outgrowth consists of an inner endoderm with a layer of splanchnic mesoderm on the outer side. The diverticulum grows rapidly to invade the extraembryonic coelom (Fig. 5.43 B1, B2) and into the space between the yolk sac, the amnion and the serosa.
Although the distal part of the allantois expands considerably, it remains connected with the hindgut by a narrow allantoic stalk. As the embryo separates itself from the extraembryonic parts, the allantoic stalk is enclosed together with the yolk stalk to form the umbilical cord. The allantois is connected with the extraembryonic circulation by paired allantoic arteries and veins.
The allantois is partly excretory in function and also acts as a reservoir by storing uric acid. It also serves as a respiratory surface by supplying oxygen to the embryo. It also helps to absorb a large quantity of albumen. With the development of allantoic circulation a considerable portion of calcium is absorbed from the egg shell.
This absorbed calcium is utilised by the embryo for the formation of bones. The egg shell by losing calcium becomes thin and delicate and thus facilitates hatching.