Problem:

I've been thinking about the development of an embryo from the zygote stage.
Required:

Question: How is it that when cytokinesis takes place at that stage, the cells all stick together in a little ball, but later in life, some are not bound to others (red blood cells, sperm cells, etc.)?

Question: How is it that some cells are stuck together and some are separated?

Question: How does this property develop?

Please explain step by step.