The cells that eventually form the organ systems of an infant start developing during the embryonic stage of prenatal development.

Here's a breakdown of the stages:

* Germinal Stage (Weeks 1-2): The fertilized egg (zygote) divides rapidly and implants in the uterine wall.

* Embryonic Stage (Weeks 3-8): This is the crucial period for organogenesis, the formation of organs and organ systems. The embryo rapidly develops a heart, brain, spinal cord, limbs, and digestive system.

* Fetal Stage (Week 9 to birth): This stage focuses on growth and development of the existing organs and systems.

Therefore, the embryonic stage is the pivotal phase for the foundation of the infant's organ systems.