Final answer:
The factor that differentiates germ line cells into spermatagonia is the SRY gene and its associated cascade. Without the SRY gene, germ cells differentiate into oogonia instead.
Step-by-step explanation:
Germ cells in the bipotential gonads differentiate into spermatogonia as part of the SRY-prompted cascade. The SRY gene recruits other genes to develop the testes, and suppresses genes that are important in female development. Without SRY, oogonia form, and primordial follicles develop in the primitive ovary.