Initially, the left and right sides of the basal plate are continuous, but during neurulation they become separated by the floor plate, and this process is directed by the notochord. Differentiation of neurons in the basal plate is under the influence of the protein Sonic hedgehog released by ventralizing structures, such as the notochord and floor plate.[1]
The basal plate (basal lamina) is separated from the alar plate (alar lamina) by the sulcus limitans (unlabeled).
John A. Kiernan (2005). Barr's the Human Nervous System: An Anatomical Viewpoint (8th ed.). Hagerstown, MD: Lippincott Williams & Wilkins. ISBN0-7817-5154-3.