Introduction: Development
This is the process by which a single cell, usually a fertilized egg, becomes a multi-celled organism. It involves the division of cells by mitosis and their differentiation into specialized cells, tissues, and organs.
Still Image:
Sea urchin egg and sperm
Hundreds of sperm surround an already-fertilized sea urchin (Lytechinus pictus) egg. The first sperm to penetrate the egg triggers formation of the fertilization membrane (most easily seen on the lower right side of the egg), which forms a barrier to other sperm.
Sea urchin eggs were fertilized, then immediately mounted in seawater between a slide and coverslip (using a silicon spacer). Images were taken at room temperature on a compound inverted microscope using a 40x DIC objectiveand digital camera.