Abstract: In one aspect, an on-chip microfluidic device (OMD) is provided for microscopic observation. In one embodiment, an on-chip gradient generating device has a silica chip having a cell loading portion configured to load the tissue. A microfluidic channel is formed in the silica chip for a chemoattractant solution having the chemoattractant to flow through, and a plurality of gradient generating ports is formed to connect to the microfluidic channel to the cell loading portion. A chemoattractant supply device is connected to an inlet of the at least one microfluidic channel for supplying the chemoattractant with a constant positive pressure to the chemoattractant solution flowing in the microfluidic channel to create the passive gradient of the chemoattractant in the chemoattractant solution such that the tissue is exposed to the chemoattractant solution having different concentration of the chemoattractant at each gradient generating port.