One of the outcomes of Hamburger’s early extirpation work in Chicago was the accidental discovery (based upon his inadvertent removal of different amount of limb bud tissue in different experiments) that there appeared to be a quantitative relationship between the amount of limb bud tissue removed and the degree of hypoplasia later observed in the lateral motor columns and the related sensory neurons. He also noted that when he transplanted limb bud tissue to other regions of the embryo (flank or belly) that hyperplasia of the motor neurons was enhanced in those regions as well [See Figure 4 from 1934 paper and NAS biog, p. 15].
In a seminal 1934 paper, Hamburger interpreted this work in the framework of Spemann’s theory of induction as his “three-point paradigm”: (1) The developing peripheral tissue (in this case limb bud) stimulates, by way of two inducer substances, the growth of motor and sensory axons toward the developing tissue; (2) The inducer substance is transported by retrograde movement from the growing neuron tip back to the cell body to stimulate further growth; and (3) The effect is quantitative: the greater the quantity of inducing tissue, the greater the rate of neuronal growth (and vice-versa, with removal of limb bud tissue). This work, and specifically the 1934 paper, laid the foundation for the studies on nerve growth factor and neuronal cell death that was to emerge in his collaboration with Rita Levi-Montalcini (1908-2012) beginning in 1947. Meanwhile, throughout the 1930s and during the war years, Hamburger carried out numerous limb-bud extirpation experiments, increasing the precision of his surgical procedures and making numerous counts of cell numbers in the spinal cord as a way of quantifying the extent of neuronal hypoplasia. It was also at this time that he initiated another set of developmental genetic studies on the creeper mutant in chickens.