When blastomeres, or groups of cells, of dividing sea urchin embryos are separated at the 2- and 4-cell stage, they develop into 'normal' but miniaturized larvae. This provides a unique system for understanding the effects of body size on animal physiology and ecology. Larvae of the urchin Arbacia punctulata were photographed under light microscopy and arranged to demonstrate how embryos from half- (middle) and quarter- (bottom) sized larvae differ morphologically from whole (top), unmanipulated larvae, having shorter arms and smaller central bodies.