In memoriam Pieter D. Nieuwkoop (1917-1996).

Pieter Nieuwkoop died September 18, 1996, at age 79, in sues formed by the flap which protruded from the neural tube. The most anterior neural structures (forebrain) develUtrecht, The Netherlands, after a brief illness. He continued his laboratory research on early vertebrate development oped at the distal end of the flap, whereas posterior structures developed at the base and matched those of the neural nearly to the end of his life. He is remembered by developmental biologists for his numerous research contributions tube level at which the implant was located. This and other experiments (by his student H. Eyal-Giladi) led him to proand integrative hypotheses over the past 50 years, especially in the areas of neural induction, meso-endoderm induction, pose that inductive neural patterning is accomplished by two factors: (1) an activating factor causing a neuralization and germ cell formation in chordates. Born in Enschede, The Netherlands, he began his doctoral of ectoderm and, if no other induction followed, leading studies shortly before World War II at the State University exclusively to anterior neural differentiation such as foreof Utrecht under the supervision of Professor Chr. P. Raven brain and midbrain; and (2) a transforming (or posteriorizing) (who had trained with M. W. Woerdeman, who had trained factor that could work only on already neuralized tissue, with H. Spemann). His thesis, written in English after the making it develop to more posterior neural parts such as war and published in 1946, concerned the determination of hindbrain and spinal cord. His was one of the first compregerm cells and the development of the germinal ridges in hensive two-component hypotheses for neural patterning; urodeles, a subject he returned to in later years. For many it came several years before Saxen and Toivonen’s double of us, acquaintance with his early work first comes with gradient model, though after T. Yamada’s rather different the Normal Tables of Xenopus laevis (Daudin) published proposals in 1950, F. E. Lehmann’s suggestions in 1942, and in 1956 with J. Faber, an enduring volume now reprinted the reports of Holtfreter and Chuang in the mid-1930s that (Garland Publishing Co.). The book contains not only their partial purification of various heterogeneous inducers gave original observations of morphogenesis and organogenesis, either a neuralizing activity or a trunk–tail-inducing activbut also a compilation of the literature on the external and ity, but not both, and that the dilution or concentration internal anatomy of embryos and tadpoles and on the breedof one did not make it act like the other. Nieuwkoop’s ing and care of frogs. During this early work Nieuwkoop activation/transformation hypothesis has survived to this and Florschütz (1950) studied Xenopus gastrulation in detail day and is cited to explain the results of contemporary exand distinguished an internal blastopore at which deep periments with pure inducers such as the noggin and mesoderm cells involute 1–2 hr before surface endoderm chordin proteins acting on isolated ectoderm. Still, it is uncells do so at the visible external blastopore. They also clear whether posterior neural tissue is always or only found that most if not all mesoderm precursors are internal formed by posteriorization of independently neuralized aneven before Xenopus gastrulation begins, and not in the terior tissue or whether it can be induced directly by a single surface layer, a disposition opposite that of urodele embryos agent (see Lamb and Harland, 1995). So great was Pieter and even other anurans, as later analyzed in detail by Ray Nieuwkoop’s familiarity with the anatomy of the amphibKeller and his colleagues. Nieuwkoop used Xenopus in his ian nervous system that he was one of the few researchers research for many years, in parallel with his use of urodeles of recent decades who could write in the methods section (axolotls, newts), which have larger and more slowly develof a paper (and have it accepted), ‘‘. . . the authors did not oping embryos more favorable for certain kinds of surgery, use molecular markers because the first author, having and his last publication cites the advantages of using both more than 50 years of experience in normal and atypical in embryology (Nieuwkoop, 1996). histology, is perfectly sure of the correct identification of all the definitive larval structures. The reliance on molecular markers [by others] has actually given rise to misinterpretaNeural Induction tions . . . in several recent studies . . .’’ (Nieuwkoop and Koster, 1995). Pieter Nieuwkoop’s first major contribution to early deIn the late 1950s he also studied the neural differentiation velopment came with his study of neural induction in uroof pH-shocked newt ectoderm and endorsed the interpretadeles in 1952 by a novel method of surgically inserting flaps tion that induced ectoderm has a self-organizing capacity of ectoderm into the dorsal midline of the neural plate of to differentiate local neural structures such as individual an early neurula embryo at different anteroposterior levels and later scoring the kinds and arrangements of neural tisbrain vesicles, despite the incoherence of the inductive sig-