Hmmm. My developmental biology is a little rusty. My basic understanding is that when a sperm fertilizes an egg cell they form a cell called the zygote which takes half its DNA from each the sperm (from the father) and the egg (from the mother). This single-cell zygote undergoes rapid divisions and cellular differentiation. This cluster of differentiating cells is the embryo. That’s about all the detail I can give, but I am sure Julia, Amy, or Kimberley might be able to give more details!
Great question bethomas, pinkcookie12, and taljeda!! Now I am anxious to learn more too =)
As Sara said a sperm cell and an egg cell fuse to form a zygote (a single cell) with all of the genetic information required for the development of a person nor animal. This zygote then divides in two and each cell in turn divides. As more cells are formed they differentiate into all of the different tissues of the body forming an embryo which continues to develop until it is able to survive outside the mother’s womb at which point it is born.
In humans the embryo is what we call a baby between the zygote stage and the foetus stage. The foetus stage is defined as 8 weeks after zygote formation – but its just a name distinction really because its the same thing just bigger and at a later developmental stage!!
In animals, the development of the zygote into an embryo goes through specific recognizable stages of blastula, gastrula, and organogenesis.
In the blastula stage the embryo is still very small! During gastrulation the cells of the blastula go through a co-ordinated process of cell division, invasion, and/or migration to form tissue layers called the endoderm, ectoderm and mesoderm. Organogenesis is the process by which your organs form – there is a lot of “talking” between the different tissue layers and cells are directed to become your liver, spine, brain, lungs etc!