It must have an "organizing principle." (Spemann - the in 1920s- did not really understand what this 'principle' was.) -> The dorsal lip of the blastopore is sufficient to organize (set up) the body axes of the embryo! Image: This transplantation experiment shows that the blastopore dorsal lip (A) induces a second embryonic axis when grafted in an area not normally expected (ectopically) to form a body axis (B). Of any portions of the gastrulating newt embryo, only the dorsal lip of the blastopore is capable of "organizing" a body axis when transplanted into an ectopic site on the same embryo. Presumptive neural ectoderm "patch" doesn't have neighboring cells -> so is never induced to become neural plate (B) Presumptive neural ectoderm "patch" does have neighboring cells -> induction by these neighboring cells allows patch to become able to form the neural plate - at the right time, the presumptive epidermis is instead induced to become a neural plate (A) Last image = neurula (embryo with neural plate -> gives rise to CNS). Why is the transplanted tissue in (B) but not in (A) able to actually induce a neural plate? Hint: That tissue previously was in contact with the overlying ectoderm. Note the difference in the age of the donors, even if the same tissue is obtained from each: presumptive neural ectoderm. What have these patches become, as a result of involution? Image: Two sets of transplantation experiments are summarized here. Prior to the induction event, inducer and inducee were neighboring patches of animal-hemisphere cells. The neural plate is induced (to form on the exterior) by the first patch of cells to involute round the dorsal lip.
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