EXPERIMENTAL FIGURE 21-37 In vertebrates, the survival of motor neurons depends on the size of the muscle target field they innervate. (a) Removal of a limb bud from one side of a chick embryo at about 2.5 days results in a marked decrease in the number of motor neurons on the affected side. In an amputated embryo (top), normal numbers of motor neurons are generated on both sides (middle). Later in development, however, many fewer motor neurons remain on the side of the spinal cord with the missing limb than on the normal side (bottom). Note that only about 50 percent of the motor neurons that are generated normally survive. (b) Transplantation of an extra limb bud into an early chick embryo produces the opposite effect, more motor neurons on the side with additional target tissue than on the normal side. See D. Purves, 1988, Body and Brain: A Trophic Theory of Neural Connections, Harvard University Press, and E. R. Kandel, J. H. Schwartz, and T. M. Jessell, 2000, Principles of Neural Science, 4th ed., McGraw-Hill, p. 1054, Figure 53-11.