3/13/2024 0 Comments Dorsal blastopore lip frog![]() The gastrulae were from differently pigmented specimen of the same species. He decided to test the state of determination of the early salamander gastrula, and to that end, he reciprocally transplanted small regions of embryos from one region on one salamander gastrula to a new region on another gastrula. He redirected his research away from the formation of specific organs (eye, ear, liver) towards looking at the problem of the earliest stages of embryonic determination. In 1914, Spemann was made head of the Division of Developmental Mechanics at the Kaiser Wilhelm Institute in Dahlem. Spemann ( 8 ) maintained that double assurance also worked for amphibian lenses, and that while some frogs had only one mechanism of lens determination, most species were in between and had some self-differentiation but still required contact for optimum lens formation.įigure 1 Hans Spemann and the lens ablation experiment B. Braus explained this phenomenon by introducing the engineering term "double assurance" into embryology ( 6 ). However, when Herman Braus removed the forelimb rudiments, the operculum still opened at the appropriate time. It had been thought that the limbs mechanically pushed through this barrier. When the limbs of amphibian tadpoles arise during metamorphosis, they emerge through an opening called the operculum. Spemann put this result in the context of a model proposed the previous year, concerning the operculum of amphibian tadpoles. There appeared to be two ways to make a lens: induction or self-determination. There was a component of induction that was species-specific. He confirmed both his earlier results and Helen King's results ( 7 ). King's paper sent Spemann back to the laboratory where he did the identical experiment on different species of frog neurulae. Spemann's and Lewis' studies were challenged again by Helen King ( 5 see 3, 6 ) who had repeated Spemann's ablation experiments on Rana palustris (a related American frog) and got the opposite results: free lenses, which were not connected to an eye. As we know, Spemann was challenged by Ernst Mencl (who claimed to get free, unattached, lenses when he did Spemann's experiment) and was supported by Warren Lewis (who had performed the ectopic grafting experiments that Spemann was trying and claimed that the optic cup could cause ectopic lens formation). Spemann's paper was exciting for several reasons, not the least being that he showed it possible to tease out the relationships between embryonic parts. He admitted that these transplantations were difficult to perform and that he was not having much success. Such certainty would come when he was able to move an optic cup into an ectopic location and observe whether lenses had been formed. Moreover, he did not know yet whether the optic cup instructed the ectoderm to form a lens or merely acted as a trigger to permit a pre-existing potency to become expressed. Spemann claimed that contact of the optic vesicle with the overlying ectoderm was needed to turn that ectoerm into a lens, but he did not know whether or not it was a sufficient cause. In those cases where the retinal rudiment had not been totally destroyed, the ability to form lenses appeared to correlate with the ability of the remnant to contact the overlying ectoderm. A few days later, Spemann observed that both the eye and the lens were missing on the operated side of the tadpole. Spemann cauterized the prospective retina anlagen in the neurula stage Rana fusca. That same year, Hans Spemann ( 3 ) published "one of the most significant and seminal papers in the history of embryology" ( 4 ), his experimental analysis of lens formation in the frog. ![]() In 1901, Curt Herbst wrote that he thought it possible "to establish the occurrence of formative stimuli which are exerted from one part of the embryo to another, and to demonstrate eventually the possiblity of a complete resolution of the entire ontogenesis into a sequence of such inductions." This prediction of inductive cascades was a bold statement, given that no sequence of events had yet been observed ( 1, 2 ).īut that was soon to change. A Selective History of Induction II Spemann's induction experiments A.
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