996 ROSE normal limb skin, the X-rayed tail does not regenerate. In addition, the tail does not suppress the regeneration of the limb skin, which can then transform to the distal part of a limb,°+! The second observation is that X-rayed tissues do not lose structure and dedifferentiate appreciably. They behave as though nothing were missing. We are beginning to understand how X rays block regeneration, First of all it is not by blocking cell division.® As many epidermal cells divide in X-rayed limbs as in normal limbs.® Thefirst clue is that limbs fail to regenerate after either X-irradiation or denervation, and they fail in the same way.! The limbs do not undergo the series of changes leading to new structures, The nerves by their presence and position determine whether and in what direction morphogenetic information will travel, If one grafts or deviates nerve bundles in worms or salamanders, a new axis of morphogenesis is established.’ For example, a piece of ventral nerve cord of the annelid, Clymeuella, grafted under and at right angles to the dorsal skin causes an outgrowth, The nature of the outgrowth, whether it be head or tafl, depends primarily upon the region in which the outgrowth occurs. A piece of anterior cord grafted to the anterior part of the body causes an outgrowth that becomes a head. Anterior cord grafted to the posterior part of the body also causes an outgrowth, but the posterior region causes it to become a tail.? The nerves set up axes along which information can travel. The region where the new axis is located is no longer controlled in the original pattern but is free to transform to the most distal structure not present along its new axis, Axes of control were demonstrated in the salamander limb by the interference resulting when two axes interacted when placed at an angle to each other.'® If two limbs were sewed together after the skin between them had been removed, both regenerated whole hands after amputation when the limbs lay parallel. If the angie between them was 30°, some of the fingers in the medial position did not regenerate, At 60° only the most lateral fingers regenerated, and at 90° all or almost all regeneration was suppressed, Monroy” reasoned that the inter- ference was linear in nature and that wherever lines crossed morphogenesis could not occur. Figure 1 shows regeneration of control doubled stumps (neither limb irradiated) and of experimental doubled stumps (right stumpirradiated), Part a of Fig, 1 shows that, when the control limbs were in a parallel position, there was no interference from the longitudinal lines of control and both stumps regenerated normally, When the stumps were at an angle, the more central lines intersected, central structures were lost, and only the lateralmost regions, where there was no interference, were able to regenerate, At very wide angles even § 4 + : t '