The Human Embryo: The First Eight Weeks Timeline

The Human Embryo: The First Eight Weeks 28

Day 0 Fertilization Human embryonic development A sperm cell fuses with the egg. The single cell that results, the zygote, carries 46 chromosomes: 23 from each parent.
Day 1–2 First cell division Human embryonic development The zygote divides into two cells, still inside its protective shell, the zona pellucida. Cleavage has begun.
Day 3 Morula: a ball of 16 cells Human embryonic development Repeated divisions produce a solid ball of cells the size of the original egg. The embryo has not grown at all.
Day 4–5 Blastocyst forms Human embryonic development A fluid-filled cavity opens. Outer cells will become the placenta, an inner cell mass will become the embryo itself.
Day 6 Implantation begins Human embryonic development The blastocyst hatches from the zona pellucida and attaches to the lining of the uterus.
Day 10 Implantation complete Human embryonic development The embryo is fully embedded in the uterine wall. The germinal stage ends.
Day 15 Primitive streak appears Human embryonic development A groove along the embryonic disc sets the body axis: head and tail, left and right.
Day 16 Gastrulation: three germ layers Human embryonic development Cells migrate through the streak and form ectoderm, mesoderm and endoderm, the source of every tissue in the body.
Day 18 Neural plate forms Human embryonic development A thickened strip of ectoderm along the back, the first trace of the brain and spinal cord.
Day 20 First somites Human embryonic development Paired blocks of mesoderm appear beside the neural tube. They will become vertebrae, ribs and muscles.
Day 21 Heart tube forms Human embryonic development Two tubes of heart tissue fuse into a single tube in the chest region.
Day 22 The heart starts beating Human embryonic development The tube begins to contract, about three weeks after fertilisation. The heart is the first organ to work.
Day 22 Neural tube begins to close Human embryonic development The neural folds meet in the middle and zip up towards the head and the tail.
Day 24 Liver bud appears Human embryonic development An outgrowth of the gut tube that will become the liver, which soon takes over blood cell production.
Day 25 Head end of the neural tube closes Human embryonic development The front opening, the rostral neuropore, seals.
Day 26 Arm buds appear Human embryonic development Small swellings on the sides of the body, the first sign of the arms.
Day 26 Lung bud appears Human embryonic development The respiratory tract branches off the front of the gut tube.
Day 27 Neural tube fully closed Human embryonic development The tail opening closes as well. Failure here causes spina bifida.
Day 28 Leg buds appear Human embryonic development The legs start two days behind the arms, a lag that persists through development.
Day 28 Eyes and ears begin Human embryonic development Lens placodes and ear vesicles form on the sides of the head.
Day 32 Five brain vesicles Human embryonic development The front of the neural tube has divided into the five regions that become the adult brain.
Day 42 Gut loops into the umbilical cord Human embryonic development The fast-growing intestine runs out of room and bulges into the cord. It returns around week 10.
Day 44 Finger rays appear Human embryonic development Ridges on the paddle-shaped hand plates mark the future fingers.
Day 47 Bone formation begins Human embryonic development The first ossification centres appear, starting in the collarbone.
Day 50 Four heart chambers Human embryonic development The walls between the chambers are largely complete. The heart has its adult layout.
Day 52 Fingers separate Human embryonic development The webbing between the fingers dies off by programmed cell death. Toes follow a few days later.
Day 54 Eyelids form Human embryonic development The eyelids grow over the eyes. They fuse shut and stay closed until about week 26.
Day 56 The embryo becomes a fetus Human embryonic development All major organs are present. About 3 cm long, the embryo is now called a fetus.