During the third week of fertilization, the zygote has three germ layers which form the trilaminar embryo. This embryo is in the form of pear shaped disk and after the formation of trilaminar disk undergoes many foldings. These foldings are the caudal folding, right and left lateral foldings and cranial folding. These foldings have effects on the embryo such as the disk like structure gets converted into cylindrical structure by the lateral foldings, the connecting stalk develops into the umbilical cord, the endoderm gets elongated and the yolk sac becomes narrow. This elongated endodermal tube is further divided into subdivisions: the part cranial to the connecting stalk becomes the foregut, the part in relation to the connecting stalk is the midgut and the part caudal to the connecting stalk becomes the hindgut.