Introduction: Fertilization
Fertilization is the process by which sex cells unite to form a zygote, or fertilized egg. Fertilization results in a new cell with a unique mix of genes. Sex cells (sperm or eggs) contain half the number of chromosomes normally found in the cells of an organism. Fertilization results in a complete set of chromosomes.
Sea urchin fertilization
Penetration by a single sperm triggers the rapid formation of the fertilization membrane around a sea urchin (Lytechinus pictus) egg.

Elapsed time about 2 minutes.
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 objective and digital camera.