Studies on the embryonic spinal cord were extended to include the development of the ascending and descending fibers from the brachial level. This study was designed to correlate morphological development of the intersegmental system with physiological and behavioral observations of early embryonic behavior. Chick embryos were analyzed prior to day 6 of incubation, which is before closure of the reflex arc and arrival of supraspinal inputs to the embryonic spinal cord. Injections of 3H-proline were made in the upper brachial level of chick embryos of stages 27--29 (5--6 days). The location of radioactive label in the marginal zone was analyzed autoradiographically to show the projection of labeled axonal flow from the site of injection. The majority of the labeled axons in the marginal zone was in the first 2--3 segments in both ascending and descending directions from the site of injection. By stage 27 (day 5), ascending labeled axons extended to the upper cervical level and labeled descending axons extended only about 4--5 segments to the upper thoracic level. By stage 29 (day 6) labeled ascending axons extended into the medulla oblongata, and labeled descending axons still extended only 4--5 segments caudally. These studies (Parts I and II) show that at stage 27 (day 5) there was considerable axoplasmic flow of radioactive label in the marginal zone from the lumbar to the brachial plexus but not in the opposite direction from the brachial to the lumbar plexus. Also, axoplasmic flow of label initially extended from the brachial plexus to the medulla oblongata sometime between stages 27--29. It was concluded that the intersegmental system of the embryonic spinal cord is well developed by day 6 of incubation and provides the neuro-anatomical substrate for early embryonic behavior.