Gastrulation in amphibian embryos is a composition of several differently located morphogenetic movements which are perfectly coordinated with each other both in space and time. We hypothesize that this coordination is mediated by biomechanical interactions between different parts of a gastrulating embryo based upon the tendency of each part to hyper-restore the value of its mechanical stress (see Beloussov and Grabovsky, 2006). The entire process of gastrulation in amphibian embryos is considered as a chain of these mutually coupled reactions, which are largely dependent upon the geometry of a given embryo part. We divide gastrulation into several partly overlapped steps, give a theoretical interpretation for each of them, formulate the experiments for testing our interpretation and describe the experimental results which confirm our point of view. Among the predicted experimental results are: inhibition of radial cell intercalation by relaxation of tensile stresses at the blastula stage; inversion of convergent intercalation movements by relaxation of circumferential stresses at the early gastrula stage; stress-promoted reorientation of axial rudiments, and others. We also show that gastrulation is going on under a more or less constant average value of tensile stresses which may play a role as rate-limiting factors. A macromorphological biomechanical approach developed in this paper is regarded as complementary to exploring the molecular machinery of gastrulation.

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Data on A's and K~ produced in central nucleus -nucleus collisions (C-C, C-Ne, O-Ne, C-Cu, C-Zr, C-Pb, O-Pb) at a 4.5 GeV/c momentum per incident nucleon obtained in the streamer chamber spectrometer SKM-200, are presented. Multiplicities, transverse momenta, rapidities and other characteristics are considered and compared with those for inelastic He-Li interactions. The polarization of A's was found to be consistent with zero. The upper limit of fl/A production ratio was estimated to be ~ 10 -2 with a 90 % confidence level. The results are compared with data of other experiments and some model calculations.

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