The beginning of the development of a human embryo begins in the ovary. Here is an unfertilized egg drifting towards the fallopian tube. It begins the fertilization process as it encounters sperm. While many may hit the egg, only one manages to deliver its genetic payload.
And when it does, the egg immediately stiffens its outer walls to prevent other entry. Over a period of about five days, the egg divides and divides again until there are exactly 32 cells, now squeezed inside the same stiffened outer wall. Inside the blastocyst, we find the group of cells that will actually develop into a human being.
The outer cells will give rise to the placenta and other supporting tissues. At this stage, it implants itself into the wall of the uterus and the pregnancy is initiated. Now we see the inner cell mass form into a disc. And at 14 days, we can see the primitive streak form, which establishes the line of bilateral symmetry.
Then the cells in the disc migrate as development continues. As the cells on one side of the disc grow faster than those on the other side, the whole disc rolls into nested tubes that will become the gut canal and other tissues. At this point, the embryo is about 28 days old and measures about 1 quarter inch long.
You can see all four limb buds and early inner ear development. Four days later, eye pits are forming and the arm buds begin to taper. Another day, and we see the beginning of nasal pits and hand plates.
This embryo is now a little less than 1 half inch long. At 37 days in, it is three quarter inches long. Nose and ears continue to develop, and thigh, leg, and foot can be distinguished.
41 days, and we can see fingers, a jaw, and a lumbar curvature. Three more days, and we have elbows and the beginning of eyelids. At 47 days, our embryo has straightened its trunk. Its limbs have extended forward and toes are visible.
Another few days, and the embryo has upper limbs slightly bent at the elbows, short stubby fingers, hands curving over towards each other, but still far apart. 52 days old and an inch long, and we see hand pads, wrists, and distinct toes. Two more days, and we see external ears, and opposite hands can now touch.
At eight weeks, the embryo is still only one and a quarter inch long. But the detail is amazing.