We investigate the motion of individual pigmented retinal cells in aggregates of neural retinal cells obtained from chicken embryos. Individual cells in aggregates move randomly in the absence of chemical or adhesion gradients: the power spectrum of position and velocity versus time correspond to Brownian motion and the velocities are uncorrelated in time. Thus a quasithermal model of cell migration like the extended Potts model is appropriate. We also measure cell diffusivity and the cell velocity distribution. The results support the idea that collections of embryonic cells behave as liquids with membrane fluctuations playing the role of temperature.