First evidence of primordial follicle can be seen in ovaries at :

At approximately 4 weeks, gonads form on the ventral surface of embryonic kidney at a site between 8th thoracic and 4th lumbar segments. Primordial follicle consists of a large cell, the primordial ovum(oogonia), which is surrounded by flattened cells,best termed as follicular epithelial cells. Reference: Shaw's Textbook of Gynaecology 15th edition, page 28