Left flesh radical (the meat-form of 肉, visually identical to 月 but distinct in meaning) - anchors 胞 in the body/anatomy family with 胃, 肝, 脏, 脑. Here it specifically signals living tissue, fitting the meanings "placenta, womb-mate, cell."
Right 包 supplies the sound - bāo unchanged. 包 also adds semantic flavour: it pictures a fetus wrapped inside a womb (勹 wrap + 巳 unborn child). With the flesh radical it gives "wrapped-flesh" - the womb, placenta, by extension the cell (细胞) and a womb-sibling (同胞 compatriot).
