Is it the seed or the soil that causes a plant to grow? A single circulating tumor cell can spawn a metastasis, but only if it has a fertile environment. 

This photograph of blood vessels (red) in a transparent zebrafish shows individual cancer cells (green) frozen in time. Researchers track their motions and interactions with their surroundings to determine how non-cancerous cells contribute to metastatic growth. Such investigations aim to identify new targets for curbing the spread of cancer through the body.