fiGURE

Four body folds cause this dramatic conversion just about simultaneously: the head fold, the tail fold, and the two lateral folds . In addition to forming the

head, the head fold brings the developing heart and mouth into their proper (adult) positions. The tail fold brings the developing anus into its proper position and, yes, human embryos do develop tails. As these two folds progress, portions of the yolk sac are incorporated into the body as the foregut and hindgut, respectively (Fig. 8-12left).