“…These conclusions apply to other species, although some species differences may be found. In the rat, with a gestational period of 21 days, the neural tube is formed relatively late at E-10; the SCO cells start to display RF-immunoreactive material at E-15 (Schöebitz et al, 1993), and the SCO is well developed at E-17 (Marcinkiewicz and Bouchaud, 1983); the first RF appears several days after birth (Schöebitz et al, 1993). Thus, rat embryos, although having a welldeveloped strongly immunoreactive SCO, lack a RF.…”