Chick Embryo Development
The chick egg has different accessory coverings around it, which are secreted, by the female reproductive tract. The following steps occur during the chick embryo development.
Fertilization takes place when the egg is entering the oviduct. Therefore, fertilization is internal. The shell is formed around the egg when it passes through the shell gland (uterus).
After the egg is laid, the development stops unless the temperature of the egg is kept nearly up to the body temperature of mother. The eggs can also be incubated artificially by regulating the temperature between 36-38°C. At this temperature development completes on 21st day and the egg is hatched.
Immediately after fertilization, a series of mitotic divisions occurs in the true egg (the so-called yolk of the egg). This is called cleavage. In chick, the cleavage occurs only in a disc-like portion of active cytoplasm present at the surface of the yolk. That is why it is called discoidal cleavage. First two divisions are vertical. The third division is horizontal which separates the disc-like cytoplasmic portion from yolk. After this, the divisions become irregular and many cells are formed. Each cell is called blastomere.
As a result of cleavage, a circular disc-like mass of cells called blastoderm is formed over the yolk. This mass consists of two or more layers of closely packed cells. The cells in the centre of disc are smaller while cells at the periphery are larger and flattened. Embryo at this stage is called morulla.
The Morula is short lived and soon changes into Blastula. Now the blastoderm splits into two layers; the upper epiblast and the lower hypoblast. The cavity between these two layers is called Blastocoele. And soon after Segmental cavity appears between hypoblast and central area of yolk due to detachment of cells from it. Now two spaces are formed, one between epiblast and hyponlast and second between hypoblast and yolk. The peripheral area of blastoderm where cells are still attached to the yolk is called zone of junction. The embryo is now called blastula.
Formation of germ layers (ectoderm, endoderm and mesoderm) by rearrangement of cells is called gastrulation. Epiblast is the presumptive ectoderm and mesoderm whereas hypoblast is the presumptive endoderm. The hypoblast grows outward around the yolk and form endodermal lining. A pool of fluid develops between central cells of hypoblast and yolk. Thus the blastoderm is raised of from the centre. At this time the central area of the circular blastoderm looks translucent (seeing from upper side) and is called area pellucida, whereas its peripheral area (zone of junction) looks like a dark ring and is called area opaca.
Now a midline thickening called primitive streak is formed on the epiblast because of inward migration of cells to form mesoderm below. This streak is formed at medial region (central region) to caudal end (posterior end) and grows in length rapidly. As a result, shape of the blastoderm is changed from circular to pear-shape. At the cephalic end (anterior end) of the primitive streak, an aggregate of cells form a thickening called the primitive node or Hensen’s node or notochordal cells. The inward migration of cells from epiblast to hypoblast continues and primitive streak changes into a groove called primitive groove. The thickened margins of primitive groove are called primitive ridges. Embryo is now called gastrula.
The cells begin to push in from the Hensen’s node to form a rod-like structure called notochord in the midline beneath the ectoderm. Notochord is visible in the chick embryo of 18 hours. The notochord is committed to form the backbone.
The marginal area (area opaca) where the germ layers (ectoderm, mesoderm and endoderm) are peripherally merging with the yolk is called germ wall. The cavity between yolk and endoderm (which is first called gastrocoele) is now called primitive gut.
The dorsal mesoderm also form group of cells called somites, which are visible in 25-26 hours embryo. Mesoderm found at the periphery of embryo is called lateral plate mesoderm. It splits into two sheet-like layers; the upper somatic mesoderm and the lower splanchnic mesoderm. The cavity between these two layers is called coelom.
On the dorsal surface of gastrula, a band of ectoderm becomes thick and is called neural plate. It is visible in embryo of 18 hours. At 21-22 hours, the neural plate folds and a neural groove is formed in the mid-dorsal line. This folding of neural plate is clearly visible at 24 hours. The embryo is now called neurula. The anterior end of the groove is widest and is the future brain whereas the rest of the portion forms spinal cord in future. Now the neural plate sinks down and the margins of the groove called neural folds grow towards each other and meet in the mid-dorsal line. As a result, the neural groove is converted into neural tube. The cavity inside the neural tube is called neurocoele and the small openings at its both ends are called neuropores. The process of formation of neural tube is called neurulation. The neural tube develops into central nervous system of chick.
POINTS TO REMEMBER
|Ectoderm||forms skin and nervous system|
|Mesoderm||forms circulatory system, reproductive system etc.|
|Endoderm||forms digestive system and respiratory system|
|Notochord||Replaced by backbone|
|Neural tube||forms brain and spinal cord|
|Somatic mesoderm||forms the future body wall|
|Splanchnic mesoderm||forms circulatory system and future gut wall|