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Two forces are necessary and sufficient to produce the transformation of the newt neural plate from a hemispheric sheet of cells one cell thick to a keyhole shape. These forces are: (1) a regionally programmed shrinkage of the surface of the neural plate (accomplished by contraction of the apical surfaces of the neural plate cells and elongation of these cells perpendicular to the plate); and (2) displacement of the whole sheet caused by elongation of either the notochord or the overlying neural plate cells in the antero-posterior direction. A computer simulation and mathematical analysis ("morphodynamics"), together with experiments and observations on embryos, were used to deduce the morphogenesis of the neural plate from these forces.

observed ionospheric peak might be an Fl region produced by solar ultraviolet radiation. It is still plausible in this case that no F2 peak will develop, and the thermal structure is not likely to differ importantly from that shown in Fig. 2 (18). A more thorough analysis of the spectroscopic data is given by M. J. S. Belton and D. M. Hunten (Astrophys. J., in press). The spectroscopic technique is discussed in a review by J. W. Chamberlain and D. M. Hunten [Rev. Geophys. 3, 299 (1965)]. 3. M. B. McElroy, J. L'Ecuyer, J. W. Chamberlain, Astrophys. J. 141, 1523 (1965); J. W. Chamberlain, ibid. 136, 582 (1962). The thermosphere models computed for the present paper differ from those reported earlier in that the adopted efficiency factors for solar heating have been reduced. Here we let only 7 ev per ion pair be converted to heat. This is about the difference between typical ionization and dissociation energies and is therefore about the minimum heating possible. It gives about one-third the total heating that was present in our previous models with N2, CO, and O. This smaller value was chosen in order to find the minimum temperatures that are physically acceptable. 4. F. S. Johnson, Science 150, 1445 (1965); a similar interpretation is to be offered by G. Fjeldbo, W. C. Fjeldbo, V. R. Eshleman. 5. Model I of C. Prabhakara and J. S. Hogan, Jr. [J. Atmos. Sci. 22, 97 (1965)] was adopted for our paper. The first Martian model in radiative equilibrium was computed by R. M. Goody [Weather 12, 3 (1957)]. Additional calculations have been made by A. Arking [Mdm. Soc. Roy. Sci. Liege 5, 180 (1963)] and by G. Ohring [Icarus 1, 328 (1963)].observed ionospheric peak might be an Fl region produced by solar ultraviolet radiation. It is still plausible in this case that no F2 peak will develop, and the thermal structure is not likely to differ importantly from that shown in Fig. 2 (18). A more thorough analysis of the spectroscopic data is given by M. J. S. Belton and D. M. Hunten (Astrophys. J., in press). The spectroscopic technique is discussed in a review by J. W. Chamberlain and D. M. Hunten [Rev. Geophys. 3, 299 (1965)]. 3. M. B. McElroy, J. L'Ecuyer, J. W. Chamberlain, Astrophys. J. 141, 1523 (1965); J. W. Chamberlain, ibid. 136, 582 (1962). The thermosphere models computed for the present paper differ from those reported earlier in that the adopted efficiency factors for solar heating have been reduced. Here we let only 7 ev per ion pair be converted to heat. This is about the difference between typical ionization and dissociation energies and is therefore about the minimum heating possible. It gives about one-third the total heating that was present in our previous models with N2, CO, and O. This smaller value was chosen in order to find the minimum temperatures that are physically acceptable. 4. F. S. Johnson, Science 150, 1445 (1965); a similar interpretation is to be offered by G. Fjeldbo, W. C. Fjeldbo, V. R. Eshleman. 5. Model I of C. Prabhakara and J. S. Hogan, Jr. [J. Atmos. Sci. 22, 97 (1965)] was adopted for o...