Other investigations have demonstrated that pressure, exerted by fluid contained within the lumen of the closed neural tube, plays an important role in the morphogenesis of cranial regions of chick embryos. In this study, we investigated whether this pressure is involved also in the formation of the secondary portion of the neural tube-the portion formed by canalization of a solid mass of cells. To determine this, the posterior neuropore was prevented from closing (resulting in myeloschisis localized to the lumbosacral region) by removing the portion of the vitelline membrane covering the embryo, and reincubating for various periods. Secondary neurulation occurred normally in embryos exhibiting lumbosacral myeloschisis, demonstrating that this process occurs autonomously in chick embryos. Additional experimental studies are needed to ascertain what specific mechanisms cause the secondary portion of the neural tube to form.