SPEAKER 1: In this video, then, we'll look at the beginning of development. Looking into the ovary at an unfertilized egg. If that gets fertilized by sperm, which travel down the fallopian tubes. So here millions of sperm coming along. Several of them will hit the egg, and try to penetrate it. But one will win, as it were. Go into the nucleus, and then there's a reprogramming process where the male and female nuclei have their genes set aside to be turned on and off for early development. Here you see early cleavage stages occurring, and this is one of the early growth phases. As the embryo moves down the fallopian tube, it's going to form an important stage called the blastocyst here in a few seconds. Of course in real life that takes days-- about five days. At this stage, then, I'd like to draw your attention to the inside of the blastocyst where there are cells called the inner cell mass, which I'll be abbreviating as ICM. Those are the cells that make the entire animal, and the outer cells give rise to the placenta and other supporting tissues. At this stage, the embryo implants into the wall of the uterus. This is when a pregnancy is really initiated. And now we'll see those blue inner cell mass cells form a disk, and then as the cells continue to grow they change their physical positions, their kind of geographical relationship to one another. And you'll see that represented here as this disk gets transformed into an embryo. Those lines represent sites where cells are migrating in and out. And here's an important stage when the three beginning layers of the embryo, the so-called germ layers are formed, and I'll come back to that in a few minutes. As development proceeds, there's more growth and movement of cells. It'll begin to form a neural tube. Here it turns, and appendages start to bud out. You see the head forming and the eye, and then eventually we get a small embryo, and some months later, of course, this would be born as a young baby.
