Inductive Processes in Embryonic Development

Abstract

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...

Evidence that the border of the neural plate may be positioned by the interaction between signals that induce ventral and dorsal mesoderm

“…One could call these early effects on the future ectodermal regions either an initial induction or a change in competence. This situation is similar to the finding that the "changing competence" of the prospective lens ectoderm is due to early induction events (Jacobson, 1966). In any event, the ventral and dorsal inductive signals could cause each of the two ectodermal regions to embark on a process of progressive determination, early stages of which may be labile or reversible.…”

Genetic interactions underlying otic placode induction and formation

“…Both the hindbrain and mesendoderm have been implicated as signaling sources for otic placode induction (Waddington, 1937;Jacobson, 1966;Chisaka et al, 1992;Moens et al, 1998;Woo and Fraser, 1998;Mendonsa and Riley, 1999), and various FGF signals are expressed in one or both of these tissues in chick, Xenopus, zebrafish, and mouse during otic placode development (Wilkinson et al, 1988(Wilkinson et al, , 1989Niswander and Martin, 1992;Tannahill et al, 1992;Mahmood et al, 1995;McKay et al, 1996;Ladher et al, 2000;Phillips et al, 2001). A role for mesendodermal FGF signaling is seen in chick, in which FGF19-expressing mesoderm can induce Wnt8c expression in adjacent neural tissue, and together these induce expression of otic markers and placode-like structures in tissue culture (Ladher et al, 2000).…”

Finger or toe: the molecular basis of limb identity

“…Similar experiments using chick embryos demonstrated that limb-type identity is determined by the origin of the graft (Hamburger, 1938): tissue taken from the wing-forming region produces an ectopic wing and cells transplanted from the leg-forming region always produce a new leg. Tissue grafts from the prospective limb-forming tissue therefore behave as classically defined developmental fields in that they have the properties of self-determination and selforganisation (Jacobson and Sater, 1988). Building upon these original observations, more extensive transplantation experiments in the chick have mapped the temporal and spatial sequence of the limb-forming capacity of the LPM (Stephens et al, 1989).…”