The developing cells before they form an embryo are called zygotes.

Here's a breakdown:

* Zygote: This is the single cell formed when a sperm fertilizes an egg. It contains the complete genetic material from both parents.

* Cleavage: The zygote undergoes rapid cell divisions called cleavage, forming a ball of cells called a morula.

* Blastocyst: The morula develops into a blastocyst, which is a hollow ball of cells with an inner cell mass. This inner cell mass will eventually become the embryo.

So, to answer your question directly: zygote is the term for the developing cells before they form an embryo.