[TA] a lateral groove running the whole length of the floor of the rhomboid fossa on either side of the midline, representing the remains of the sulcus demarcating the alar (dorsal) from the basal (ventral) plate of the embryonic rhombencephalon; position of sulcus indicates the general separation of motor nuclei of cranial nerves (located medially) from sensory nuclei of cranial nerves (located laterally). SYN: sulcus limitans ventriculi quarti [TA].