For simplicity, we adopt the following convention: a, b, c, d, r1, r2, r3, r, r4, s1, s2 are real numbers, p, q are points of E 2 T , P is a subset of the carrier of E2 T , and X, Y , Z are non empty topological spaces. Next we state a number of propositions: (1) For all a, b, c holds c ∈ [a, b] iff a ¬ c and c ¬ b. (2) Let f be a continuous mapping from X into Y and g be a continuous mapping f...

Within the developing vertebrate head, neural crest cells (NCCs) migrate from the dorsal surface of the hindbrain into the mesenchyme adjacent to rhombomeres (r)1 plus r2, r4 and r6 in three segregated streams. NCCs do not enter the intervening mesenchyme adjacent to r3 or r5, suggesting that these regions contain a NCC-repulsive activity. We have used surgical manipulations in the chick to dem...

Observations of knock-out mice suggest that breathing at birth requires correct development of a specific hindbrain territory corresponding to rhombomeres (r) 3 and 4. Focusing on this territory, we examined the development of a neuronal rhythm generator in the chick embryo. We show that rhythmic activity in r4 is inducible after developmental stage 10 through interaction with r3. Although the ...

We have previously demonstrated that inactivation of the Krox20 gene led to the disappearance of its segmental expression territories in the hindbrain, the rhombomeres (r) 3 and 5. We now performed a detailed analysis of the fate of prospective r3 and r5 cells in Krox20 mutant embryos. Genetic fate mapping indicates that at least some of these cells persist in the absence of a functional Krox20...