developed of embroy in hen
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The chicken (taxon -Gallus gallus)embryo develops and hatches in 20 to 21 days and has been extensively used in embryology studies. ... Fertilized eggs can be easily maintained in humidified incubators and during early stages of development the embryofloats on to of the egg yolk that it is using for nutrition.
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Fertilisation and early development
Fertilisation of the germinal disc by the sperm takes place in the infundibulum about 15 minutes after its holding follicle has released the yolk. Cell division to create the new embryo starts about five hours after fertilisation and continues while the egg passes along the oviduct and after the egg is laid. It is generally said that the hen’s egg takes 21 days of favourable incubation conditions for the chicken to develop and hatch. However, this development takes 22 days – one day in the oviduct and 21 days in the incubator or nest.
The zygote
When the sperm cell (with half the required chromosomes) fertilises the female egg cell (with the other half of the required chromosomes) it forms the zygote, which is a single cell with the correct number of chromosomes. About five hours after fertilisation the zygote enters the isthmus and it is here that the new embryo starts to develop by simple cell division. By the time the egg leaves the isthmus, the zygote, now called the blastoderm or embryo, comprises eight cells and after four hours in the uterus it has grown to 256 cells.
Early development of the chick embryo (2 days)
formation of ectoderm, endoderm and mesoderm
Initially the dividing cells form one layer over the yolk, but as cell division continues two layers are formed. These are called the ectoderm (uppermost) and the endoderm (underneath) layers. At about this stage the central cells of the blastoderm separate from their contact with the yolk to form a cavity. It is in this cavity that subsequent embryo development occurs. Soon after the formation of the ectoderm and endoderm, a third layer of cells called the mesoderm, or middle layer, is formed.
From this stage on, the organs and tissues of the bird will develop from these three layers of cells.
The ectoderm produces the nervous system, parts of the eyes, the feathers, beak, claws and skin.
The endoderm produces the respiratory system, the digestive system and secretory organs.
The mesoderm produces the skeleton, muscles, circulatory system, reproductive organs and excretory system.
Another important development at this stage is the way the cells change to allow the production of the different types of cells that make up the tissues. By the time the egg is laid the embryo consists of many cells differentiating into the various tissues, organs and body systems.
Physiological zero
The fowl retains some vestiges of the characteristics of its reptilian ancestors. One characteristic in particular is the influence of ambient temperature during the post laying period on embryonic development. When the temperature of the egg is below 20°C, the embryo becomes dormant and most development stops. When the temperature rises above 20°C, embryonic activity re-initiates. This temperature of about 20°C when embryonic activity starts or stops is often referred to as a physiological zero.
Fluctuating temperatures above/below 20°C will create a start/stop response in embryonic development, and each succeeding response progressively weakens the embryo. The temperature must be increased to the required 37-38°C for optimum development to occur. Failure to satisfy this need leads to significantly weaker embryos. To retain maximum viability of the embryo, hatching eggs should be processed and placed in cool storage below 20°C as soon as possible after collection and held at that temperature until the pre-warming process just prior to setting the eggs in the incubator. Once in the incubator, the temperature must be controlled within very close parameters.
Extra embryonic membranes of the chicken
Extra-embryonic membranes
Because the avian embryo has no anatomical connection to the hen, all of its nutritive requirements, except oxygen, must be contained within the egg. From very early on, the embryo develops special membranes external to its body to access the nutrients in the egg and to carry out essential bodily functions.
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