The circulatory system is one of thefirst to function during development. The earliest event in thesystem's development is vasculogenesis, whereby vascularprogeniter cells form clusters called blood islands, which laterfuse to form capillary networks. There exists a very goodin vitro system that mimics this process. When HUVECs(Human Umbilical Vein Endothelial Cells) are cultured on Matrigel,they spontaneously form a capillary network structure. Twotheoretical models have been proposed to explain the patternformation of this in vitro system. Both models utilizechemotaxis to generate spatial instability, and one modelspecifies VEGF as the chemoattractant. However, there are severalunknown factors concerning the experimental model. First, thepattern formation process occurs at the interface between theliquid medium and Matrigel, and it is unclear whether diffusion inthe liquid or gel is critical. Second, the diffusion coefficientof VEGF, which determines the spatial scale of the capillarystructure, has not been properly measured. In the present study,we modified the experimental system to clarify the effect ofdiffusion in Matrigel, and experimentally measured the diffusioncoefficient of VEGF in this system. The relationship with thespatial scale of the pattern generated is discussed.