Embryo Development 

Simply Amazing! Mother Nature, at one of her finest moments, creates a chicken out of an egg. Have you ever wondered just what happens inside the egg and when? It takes 21 days and a lot has to happen in that amount of time! Here’s a bit of training in embryology so you will know. By the time the egg is laid, some of its development has begun, but in order for it to continue on into becoming that fluffy little chick, certain conditions must be present. The temperature and humidity must be right, then it’s on with the show! Shortly after the incubation process begins, things start to happen. First, a pointed thick layer of cells becomes visible in the caudal or tail end of the embryo. The pointed area is the primitive streak, where the head and backbone of the embryo will develop. Other things are happening too. Blood islands appear that will later develop into the blood system. The eye begins to develop and the precursor of the digestive tract forms. Between the 24th and 48th hour of incubation, the heart is being formed while the blood islands begin linking to form the vascular system. By the 44th hour, the heart and vascular system join, and the heart takes it’s very first beat. Two separate circulatory systems are established; one for the embryo and one, a vitelline (meaning yolk) system extending into the egg. After 36 hours of incubation, the beak begins it’s development and limb buds begin to form that will become the legs and wings later. The embryo continues turning and twisting through the fourth day, until the chick’s entire body has turned 90 degrees and lies down with it’s left side on the yolk. Finally, it comes to rest in a ‘c’ shape as the head and tail come close together. Parts of the digestive and respiratory systems develop, as well as the mouth, tongue, and nasal pits. The heart has yet to be enclosed within the body, but it has gotten larger and can be seen beating if the egg is opened carefully. Internal organs have been developing and by the end of the fourth day all organs that the chick will need to live outside the egg can be identified. At this point in incubation, the embryo cannot be distinguished from that of mammal embryos. Growth and development progress rapidly, by day 7 digits appear on the wings and feet. The heart is now enclosed in the body of the chick and the embryo has begun to look like a bird. Halfway through incubation, feathers and feather tracts are visible and the beak hardens. By 14 days, claws are forming and the chick is moving into position for hatching. At 20 days of incubation, it is in hatching position and the beak has pierced the air cell. Some time on the 21st day, the chick will begin to break away from its home up to this point. It begins by pushing its beak through the air cell and using its own lungs to breathe. First, just a crack in the shell is all that you will notice, but as the chick uses its egg tooth and neck muscles it begins to break a hole in the shell. The chick will then rest a bit, change position and start back on breaking away pieces of the shell. This cycle of chipping, resting and turning in the shell will continue until at last the top part of the egg is completely free from the rest of the egg. After a short rest the chick will begin to push the top off and the head will be free at last. The final struggle will be getting the rest of its body free. The chick is exhausted and will rest as the naval heals and its down dries. The hatching is completed and as it regains some strength it will begin to walk. Events in Embryonic Development Before being laid: Fertilization Division and growth of living cells Segregation of cells into groups of special function (tissues) Between laying and incubation: No growth; inactive stage During incubation: *First Day 16 hours – first sign of resemblance to a chick embryo 18 hours – appearance of alimentary tract 20 hours – appearance of vertebral column 21 hours – beginning of nervous system 22 hours – beginning of head 24 hours – beginning of eye *Second Day 25 hours – beginning of heart 35 hours – beginning of ear 42 hours – heart beats *Third Day 60 hours – beginning of nose 62 hours – beginning of legs 64 hours – beginning of wings *Fourth Day beginning of tongue *Fifth Day formation of reproductive organs and differentiation of sex *Sixth Day beginning of beak *Eighth Day beginning of feathers *Tenth Day beak begins to harden *Thirteenth Day appearance of scales and claws *Fourteenth Day embryo gets into hatching position *Sixteenth Day scales, claws and beak becoming firm and horny *Seventeenth Day beak turns toward air cell *Nineteenth Day yolk sac begins to enter the body cavity *Twentieth Day yolk sac completely drawn into body *Twenty-first Day hatching of chick Resource for article information came from Mississippi State University Extension Service Gander Gab/PerfectPets