In the second week of development the dividing zygote forms a solid ball of cells and is now called a blastocyst. The cells on the surface of the ball will not form part of the embryo and will go on to make the placenta and membranes that surround the embryo. Inside the ball of cells a fluid filled space forms beside which is a small clump of cells. This small clump of cells (embryoblast) will form all the parts of the embryo.
During this second week the blastocyst will stick to the inside of the uterus and will no longer float free. This sticking process is required to allow the blastocyst to begin to get food from the mother. (You start as a single cell, how many cells are you now?)
Go to Week 3 the ball of cells forms first connections with mother.
Most of the early important stages of embryo growth occur before the mother knows she is pregnant.
Movie of Human Embryo Growth (this shows a human embryo growing, all images are to scale)
Movie of Limb Growth(this shows development of the human arm from bud to paddle, to hand with arm)
Movie of Mouse Embryo Growth (this shows a mouse embryo growing)
Movie of Rotating Embryo(this shows an early embryo from all angles)
All Moviesthis shows many different ways of looking in at changes in different embryos.