1. As tissue (X) develops, it secretes something that slows down the growth of tissue (Y)
2. Tissue (X) secretes RNA which changes the development of tissue (Y)
3. As tissue (X) develops, it secretes enzymes that inhibit the development of tissue (Y)
4. As tissue (X) develops, it secretes something that induces tissue (Y) to develop

During embryonic development, the primary organiser signals the development of another organ or tissue by secreting chemical factors.