Development

• Embryonic Development
• The Development of the Chick

The Development of the Chick
(Campbell 6th Ed. 1009-1010; 7th Ed. 998-999)

The chicken egg consists of the ovum, commonly called the yolk, and the accessory materials, namely albumen, shell membranes and shell. The yolk or ovum is formed in the ovary and discharged into the abdominal cavity near the opening of the oviduct. The ovum is swept by ciliary action into the oviduct where fertilization may occur. The egg cell or ovum released from the hen's ovary consists of the yellow yolk mass plus a small area of yolk-free cytoplasm, the germinal disc, which is located on the surface of the yolk. Because of large amounts of yolk, cleavage occurs only in the germinal disc and the embryo grows by absorbing raw materials for its metabolism from the yolk.

During its passage down the oviduct, several accessory features are added to the ovum (note: the term "egg" is best reserved for the shelled entity laid by the hen, while the egg cell, that structure released from the ovary, is called an "ovum"). In the first section of the oviduct, a viscous stringy albumen is secreted and adheres to the ovum. As the ovum moves along, rotation twists the stringy albumen into a pair of strands which surround and project beyond the ovum. In the next oviduct region, a more- watery albumen is added. Finally, two shell membranes and a calcareous shell are applied enclosing the entire structure.

The egg is a remarkable piece of architecture. The hard protective shell is porous, permitting gas exchange with the environment. Albumen serves as a water reservoir for the early embryo and a source of food for the later embryo; it also contains substances that retard the growth of bacteria. The yolk of the ovum contains proteins, carbohydrates, lipids and vitamins that sustain the metabolic needs of the rapidly growing young chick during its normal 21-day incubation. The yolk is surrounded by a delicate vitelline membrane. A small mass of active protoplasm, the germinal disc lies just under the membrane and can be easily seen as a small grey spot. In unfertilized eggs, it contains a haploid nucleus. Twisted, dense albumen strands extend from the yolk to each end of the egg and are known as chalazae. They center the yolk within the albumen and provide protection.

In fertilized eggs, cleavage occurs in the germinal disc either while the egg descends the oviduct or while it is retained in the cloaca. By the time it is laid, it has reached the gastrula stage and appears as a flat patch of cells called the blastoderm.

24 Hour Chick Embryo

After cleavage of the blastoderm, it separates from the underlying cavity, the blastocoel. In gastrulation, the blastoderm becomes two-layered, the endoderm lying ventrally and the ectoderm on the surface. The mesoderm arises by proliferation of cells laterally between the ecto- and endoderm on either side of the primitive streak.
Note primitive streak, neural groove and somites.

Examine the 24 hour chick embryo:

1. Prepared Slide
2. Micrograph
3. Live Specimen

48 Hour Chick Embryo

Some of the prominent features include primitive streak, somites and neural groove (note that the notochord lies immediately beneath the neural groove).

Examine the 48 hour chick embryo:

1. Prepared Slide
2. Micrograph
3. Live Specimen

72 Hour Chick Embryo

The embryo has a distinct C-shape at this stage. It is covered by a thin- walled sac, the amnion. Note the development of the somites. The embryo has well-developed circulation at this stage. Observe the heart and its chambers. The smaller, slightly more anterior portion is the atrium and the larger, more ventral portion is the ventricle. Observe the auditory vesicles and optic cups.

Examine the 72 hour chick embryo:

1. Prepared Slide
2. Micrograph
3. Live Specimen

4 Day (96 hour) Chick Embryo

Extending out from the embryo is a thin delicate membrane with blood vessels at its surface. This membrane, called the allantois, expands as the embryo develops until it completely lines the shell. The transparent sac surrounding the embryo is the amnion. The material in the amnion is principally water from the egg white. The embryo is actually developing in the amniotic fluid. Look for limb buds. Examine the eyes and compare their size with the size of the embryo. The large eyes are a characteristic that separates bird embryos from all other vertebrate embryos at the same stage.

Examine the 96 hour chick embryo:

1. Prepared Slide
2. Micrograph

Live Five Day Chick Embryo

Find all of the above (96 hour and 72 hour) structures in the five day embryo.

Examine the live 5 day chick embryo
Examine the live 5 day chick embryo up close