Signal Transduction![Figure][1] Frog eggs are used to observe cell signals. CREDIT: ROSEMARY CALVERT/GETTY IMAGES When diffusion is too slow for communication over long distances, cells can use waves of chemical activity. By using fluorescent probes and microscopy, Cheng and Ferrell show that in frog eggs (which are very large cells), waves of apoptotic signals can be seen passing through the egg cytoplasm. The pathways that trigger cell death have positive feedback loops that lead to self-regenerating waves. The speed of the waves (∼30 micrometers per minute) is too fast to be explained by diffusion. Science , this issue p. [607][2] [1]: pending:yes [2]: /lookup/doi/10.1126/science.aah4065