Tail regions from chick embryos at three to ten days of incubation were examined by light and electron microscopy to determine what morphogenetic processes occur during transformation of the embryonic tail into the definitive type. The embryonic tail attained its maximum length (0.6 mm) between four and five days of incubation. Thereafter, the tip and base of the tail developed differently. The diameter of the tip of the tail decreased during four to six days of incubation, and many macrophages and presumptive necrotic cells appeared in this area. The tip of the tail then gradually increased in diameter during six to ten days of incubation, and changed in shape from conical to pyramidal. Only a few macrophages and presumptive necrotic cells were present during these latter stages. In most embryos during seven to ten days of incubation, the caudal end of the neural tube was cystic and small hemorrhagic zones were present in the ventral part of the tip of the tail. Near the end of this period, the ectoderm overlying the cystic portion of the neural tube often ruptured. The base of the tail increased gradually in breadth during all stages. Between six and ten days of incubation, as the leg buds elongated rapidly, the cranial part of the base of the tail became incorporated into the caudal part of the trunk. These results suggest that during morphogenesis of the tail region three morphogenetic events occur: differential growth, incorporation of part of the tail into the trunk, and cell death. The definitive tail of late incubation and posthatching stages is derived from mainly the distal portion of the base of the embryonic tail; the tip of the embryonic tail is mostly lost.