ABSTRACT
A1 D The blastocyst forms primordial cells in week 1 of gestation. At week 3, cells form on the wall of the yolk sac. Actual migration of totipotential primordial germ cells does not occur until weeks 5-6, when cells migrate from the yolk sac along the mesentery to the gonads and then become gonadal or extragonadal. Abnormal migration may give rise to midline tumours, which mainly occur in the cervical, retroperitoneal, sacrococcygeal, mediastinum and pineal gland.
