Germ cells in the ovary develop from ?

C i.e. Yolk SacUmbilical vesicle (Yolk sac) attains full development & is large at 32 days (4-5 weeks)Q. It can be observed sonographically early in 5th week. By 10 weeks, it shrinks to pear shaped 5 mm diameter and after 20 weeks it is not visible.Although umbilical vesicle contains no yolk, its essential due to- Site of origin of primordial germ cells is endodermal lining of wall of umbilical vesicleQ (in 3rd week)- In 3rd week blood first develop in vascularized extraembryonic mesoderm covering the wall of umbilical vesicle, and continues to form there until hemopoietic activity begins in liver during 6th weekIts endoderm, derived from epiblast, gives rise to epithelium of trachea, bronchi, lungs and digestive tract (form primordial gut in 4th week)Transfer nutrients to embryo in 2nd & 3rd weeks