After plating the neurons look okay but they all migrate and clump at the edge of the plate. How can I keep them at the center of the well? I am using the 6-well plate. I was told to place a drop of 0.5 ml of resuspended cells at the center, but last time I did so the drop got dispersed when I was moving the plate. I am worried that there may not be enough medium to cover all neurons.