On the right is a single fertilised egg — the zygote. Press ▶ Play and watch it divide: 1 → 2 → 4 → 8 cells. These are cleavage divisions, rapid mitosis in which the cells split but do not grow in between — so the ball gains cells without getting bigger. The solid ball of cells is the morula. ⏭ Step to take it one division at a time.
Keep playing. After the morula, the cells move to the outside and a fluid-filled cavity opens in the middle. The solid ball becomes a hollow ball — the blastula. Its cavity is the blastocoel; in mammals this stage is the blastocyst that implants in the uterus. ↻ Reset and ▶ Play to watch zygote → morula → hollow blastula in one run.
Switch to the Germ layers tab. Press ▶ Play and watch the ball fold inward — gastrulation. Three labelled sheets appear: the outer ectoderm, the middle mesoderm, and the inner endoderm. These three germ layers are the foundation from which the whole body is built. ⏭ Step to reveal them one at a time.
Now the key exam fact. ⏭ Step through and watch each layer light up with what it becomes: ectoderm → skin & the nervous system; mesoderm → muscle, bone & blood; endoderm → the lining of the gut & lungs. Remember it as outer = skin/nerves, middle = meat (muscle/bone/blood), inner = linings.
Open the Development timeline tab. Press ▶ Play to sweep along the journey: zygote → embryo → foetus → birth. Watch the marker pass through organogenesis, where the germ layers fold into organs; by about 8 weeks in humans the major organs are present and the embryo becomes a foetus. ⏭ Step to stop at each milestone.
Keep playing past birth and the curve becomes the aging curve: body function climbs to a peak, then slowly declines — this is aging, or senescence. The cellular causes are telomere shortening at every division and accumulating wear & free-radical damage until repair can't keep up. ↻ Reset and ▶ Play to ride the whole life curve. Revisit any of it in the Lecture.