A precursor of the digestive tract forms blood islands appear and will develop later into the vascular or blood system and the eye begins. From the primitive streak, the head and backbone of the embryo develop. This pointed area is the primitive streak, and is the longitudinal axis of the embryo.
Soon after incubation begins, a pointed thickened layer of cells becomes visible in the caudal or tail end of the embryo.
Some cells may become vital organs others become a wing or leg. At first, all the cells are alike, but as the embryo develops, cell differences are observed. When the egg is laid, some embryonic development has occurred and usually stops until proper cell environmental conditions are established for incubation to resume. The complexity of the development cannot be understood without training in embryology. A chick emerges after a brief three weeks of incubation. One of the greatest miracles of nature is the transformation of the egg into the chick.
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