Background about the development of a bird
The development of a bird from an egg is the complex and speedy process which takes 21 days from the moment the embryo forms. The developments are facilitated by among other factors the presence of pores (about 7,000 of them) on the egg's surface. The pores allow transfer of gasses (oxygen diffuses in while carbon dioxide and water vapor diffuse out) through the shell (Gilbert). Before the egg is laid, it is fertilized; the living cells divide and grow wile segregating into groups. During incubation, the first sign of an embryo appears at 13 to 16 hours after the egg is out under warm conditions conducive for hatching (Gilbert). Other parts such as the alimentary tract, vertebral column, nervous system, the head and the eye form within the first 20 hours! Progressively after one day, the heart, the ear, and the heartbeats come into play into the second day followed by the nose, legs and wings on the third day. Progressively, other parts are formed within two days with several formations taking place one after the other until the 21st day when the mature chick hatches.
13-hour chick
At 13 hours the embryo looks like a string with a node somewhere in the middle. The node is called the Henson node. Other parts are the endoderm, mesoderm, yolk at the end, and the ectoderm at the extreme opposite end.
24-hour chick
At 22 hours, the neural folds and the somites can be noticed. The neural folds tend to close at the midbrain section to close up on the brain while they remain open everywhere else. The somites appear as blocks of mesoderm along the notochord and slide over at the underneath part. Meanwhile, the embryo folds up off the blastodisc while the gut closes from the yolk sac. Notably, the primitive streak regresses towards the posterior end of the embryo.
48-hour chick
At this stage, the embryo starts to twist due to torsion while the brain starts to get flexed by the cervical and cephalic flextures. The amnion starts to cover the embryo as it lies on its left side. The eyes can be noticed to have formed as seen by the optic vesicles invaginating to form a cup while the lens placode invaginates to form a lens that fits the front end of the optic cup. Moreover, the brain can be seen evidently by the presence of the fore, mid and hind-brain. Also, the heart can be seen as an S-shaped tube while the four chambers can be seen as four packs on each other. The most posterior end of the heart-sinus venosus is the place where the veins enter. Other parts of the heart such as the atrium come after the Sinus Venosus.
72-hour chick
At this stage, the sinus venosus is being incorporated to become a part of the right atrium. The SV later becomes the heart’s pacemaker because its cells have the fastest intrinsic beat. The valves are visible at the junction. Other visible parts include the aortic arches and a large anterior cardinal vein which goes on to become the vena cava with its principal purpose being to take blood to the brain.
Somites
Somites refer to each of some body segments containing the same internal structures, clearly visible in invertebrates such as earthworms but also present in the embryonic stages of vertebrates.
Functions of the different parts of a bird embryo
Egg shell- offers protection to the embryo and is permeable to allow for the exchange of carbon dioxide and oxygen out and into the egg respectively.
Albumin or egg white- it provides the embryo with proteins and water. The fatty egg yolk provides energy to the embryo.
Chorion- it is not part of the body of the developing embryo and its function is to facilitate the exchange of carbon dioxide and oxygen between the embryo and the shell and overall external environment.
Amnion- protects the embryo from mechanical shocks. It also supports hydration
Allantois- it stores all the nitrogenous wastes that the embryo produces. Moreover, it supports respiration. (Gilbert)
Neurulation is the process in vertebrates which results in the formation of the neural tube giving rise to the development of the brain and the spinal cord. In the process of neurulation, neural crest cells are formed, and they move away from the neural tube to form cells responsible for pigmentation as well as neurons.
Organogenesis refers to the period in the development of an embryo when it becomes a fully functional organism that can survive on its own. In-depth, it refers to the process of the formation of organs characterized by cell differentiation and cell movements (Gilbert). Gastrulation is the process of bilaminar and trilaminar germ disc formation which is the next stage after the cleavage in the embryogenesis.
Works cited
Gilbert, S. F Developmental Biology, 6th edition. Sundeland (MA). Sinauer Associates. 2000. Print
