Answer:
The zygote is formed when two sex cells join to become haploid. It is at this stage that the zygote begins to split and make more cells. It keeps doing this for 5 days until it is called a blastocyst, which is a ball of cells. After this point, the blastocyst is called an embryo or fetus. The embro continues to make cells which eventually become specialised, like skin cells and muscle cells.
It doesn't have anything to do with conservation of mass, just splitting and making thousands of new cells until the zygote becomes a baby.