@staticmethod
def basal_apical_axis(eptm, cell):
    """
        Returns a unit vector allong the apical-basal axis of the cell
        """
    edges = eptm.edge_df[eptm.edge_df['cell'] == cell]
    srce_segments = eptm.vert_df.loc[edges['srce'], 'segment']
    srce_segments.index = edges.index
    trgt_segments = eptm.vert_df.loc[edges['trgt'], 'segment']
    trgt_segments.index = edges.index
    ba_edges = edges[(srce_segments == 'apical') & (trgt_segments == 'basal')]
    return ba_edges[eptm.dcoords].mean()
