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...