What controls the development of cells and tissues in multicellular organisms?

Answer 1

Differential gene expression as a cell differentiates from a stem state to a specialized one. The way that genes are expressed changes as a cell ages. It also has to do with cell-cell or environmental signaling. After fertilization (when an egg and sperm unite to create a zygote), there is a series of rapid cell divisions which results in a hollow ball of cells with 3 germ layers (mesoderm, endoderm, and ectoderm). The cells in these three layers later differentiate into all the tissues and organs of the body.