Zygote and blastomeres; later trophectoderm and the darker inner cell massZona pellucida; holding pipetteA human embryo at true scale (ruler: 1 tick = 10 µm): a zygote about 110 µm across in a zona pellucida about 15 µm thick, held by a fire-polished holding pipette as in an IVF lab. Cleavage divides without growth, so every division halves cell volume and cells shrink to 0.79× their mother's radius. From the 4-cell stage the divisions drift out of step. Around day 3–4 the cells compact: they flatten against each other and their borders fade. From day 4.5 the outer cells pump sodium inward, water follows, and a cavity opens: the trophectoderm becomes a shell (its near half cut away here) and the inner cell mass, the future embryo proper, gathers at one pole. By day 5.5 the blastocyst swells and thins the zona before hatching. Timings are typical IVF averages; real embryos vary by hours.
~110 µm人类合子（不含透明带）human zygote (without zona)×0.79每次卵裂后细胞半径（体积减半）cell radius after each cleavage (half the volume)day 3–4致密化（8–16 细胞）compaction (8–16 cells)day 5囊胚，约 70–100 个细胞blastocyst, ~70–100 cells~20–30%内细胞团所占比例share in the inner cell massday 6–7孵化出透明带、着床hatching from the zona, implantation