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Hans Spemann won the Nobel Prize in Physiology or Medicine in 1935 for conceiving of and demonstrating the Organizer Effect in Embryonic Development. He and Hilde Mangold identified and removed a region of the early amphibian embryo that had the capacity to induce an axis and grafted it into another embryo such that both the normal host and graft-induced axes would form, creating conjoined twins. Here that experiment is replicated by Dr. Eddy DeRobertis of the Howard Hughes Medical Institute at UCLA. Briefly: In one of two embryos, the dorsal blastopore lip, the organizer, is clearly visible as a crescent. A square of organizer tissue is excised, with the help of a tungsten needle and forceps, in a freehand operation. The organizer is pushed into the ventral side of a recipient gastrula stage host embryo. One hour after transplantation, the graft has healed in the host embryo. Two days later, a Siamese twin with two perfect body axes is seen swimming. The organizer graft induced complete central nervous system and mesodermal somites in tissues of the host that would otherwise have become ventral tissue.
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