posted February 26, 7 pm
a) What major discovery was made by Hans Driesch?
b) What stage of development of what kind of animal was Driesch studying when he made this discovery?
c) Are other any other kinds of animals in which this same phenomenon has also been discovered?
d) Are there any vertebrates in which this phenomenon occurs to an even greater extent that Driesch discovered?
e) Does this phenomenon also occur in Dictyostelium?
f) Are there kinds of animal in which the phenomenon discovered by Driesch occurs to a much smaller degree than in vertebrates?
g) What is a specific example?
h) What do the words "mosaic" and "regulative mean in the context of early embryonic development?
i) What result did Driesch expect to observe?
j) What earlier experiments by Roux supported expections much different from what Driesch observed?
k) If Driesch had done his experiments using nematode embryos, would he have been surprised by the result?
l) Driesch's discovery is related to what kind of symmetry?
m) How is Driesch's concept of "Entelechies" related to his observations?
n) Has anyone ever figured out the true explanation for the results of Roux's "hot needle experiment"?
o) Can you suggest a cause for what happens in the hot needle experiment?
p) Suggest how to do equivalent experiments using
* mammal embryos?
* sea urchin embryos?
* Dictyostelium slugs and fruiting bodies?
(Also please suggest better ways of damaging cells than poking them with red-hot needles.)
q) Assuming that having been poked with a red-hot needle would decrease cells' secretion of "morphogen" chemicals, then sketch and explain the expected results of Roux experiment in embryos that use Turing's "reaction-diffusion" type of pattern-generating mechanism.
r) If embryonic patterns are controlled by the kinds of linear diffusion gradients that Lewis Wolpert hypothesized (and if hot needles reduce secretion of "morphogen" chemicals), then how would the results of Roux hot needle experiment differ from what actually happens?
s) Do you understand why Wolpert's "French Flag" model was invented? To provide a hypothetical explanation for what result?
t) Please try to invent a hypothesis that accurately predicts the results of Roux observations and also predicts what Driesch discovered.
u) If you push two one-cell-stage embryos together side by side, in echinoderms and mammals, both cells will develop (often) into half an embryo. Imagine a series of experiments in which very thin, semi-permeable membranes are positioned between the two cells, so that whatever signals travel from one side to the other have to diffuse through this membrane. Membranes can be bought that are permeable only to molecules smaller than particular molecular weights (e.g. only molecules smaller that 1,000 amu., and other membranes only permeable to molecules smaller that 10,000 amu.
What results would you try to find; and what could those results prove (support) about the mechanisms that control early development?
v) In order to continue the Roux hot-needle type of experiment, why would you be helped by using a species of animal that lays and fertilizes its eggs one or two at a time, at intervals of about an hour, starting about 10 in the morning and continuing until about 5,
and repeating this time sequence every day?
(Hint, why is it a problem if your experimental species lays eggs in the middle of the night on weekends, all its eggs at once, and unpredictably?)
w) Explain why you would (or would not) regard embryonic regulation as an example of homeostasis?
x) Does homeostasis create properties, or just prevent properties from changing. (or both)?
y) Are reaction-diffusion mechanisms, that Turing invented, special kinds of homeostatic mechanisms?
z) Is Turing's theoretical mechanism based on positive feed-back or negative feed-back?
#) Sketch and explain the punching dummy analogy that is mentioned so often in the lectures.
?) What are similarities and differences between homeostasis and the minimization of thermodynamic free energy?