The processed single cell RNA data from E10-E18, P1 and P4 from Di Bella & Habibi, et al. can be downloaded from the Broad Single-cell Portal (https://singlecell.broadinstitute.org/single_cell/study/SCP1290/molecular-logic-of-cellular-diversification-in-the-mammalian-cerebral-cortex).
URD reqires to define the 'root' cells (the base of the specification tree), from the first developmental stage and the 'tips' (i.e. the terminal populations) of the tree. You can load these cells as follow:
load("root_and_tips.obj")