Abstracttrans,trans,anti, trans,trans-Perhydropyrene, ( 1,2,3,-3ar,4,5,5at,6,7,8,8at,9,10,10ac, lographically independent molecules in the asymmetric unit are tilted with respect to each other at a dihedral angle of 53-9" the crystal structure is of a layer type with hexagonal packing. Upon heating, a phase transformation occurs at 344.5 K which is associated with jumps of the crystals almost 6 cm high. The transformation is probably of the martensitic type with lateral shifts of lattice planes relative to each other.

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Amphibian gastrula ectoderm was isolated and examined by scanning electron microscopy after 24 h, 48 h, 6 and 10 days in culture.Contrary to the view which was formerly held, ectoderm is already detemined to form epidermis at the early gastrula stage. The ultrastructural differentiation of the epidermal cellsin vitro is very similar to their developmentin vivo (Billettet al., 1973). The surface of the individual epidermal cell is curved convexly like a hemisphere and is covered by numerous microvilli. About 1 in 10 of the epidermal cells is ciliated.Ectoderm treated with vegetalizing (mesodermal/endodermal inducing) factor for 24 h differentiates predominantly into endodermal tissues. Already 24 h after the implantation of the inducer (employing the sandwich-technique) there is an obvious difference between the test series and the controls. The surface of the cells is flattened and covered by projections resembling processes of antlers. After 10 days (culture the cells with flat surfaces are arranged like an irregular honeycomb. Ciliated cells cannot be detected.The morphological differences in the surface of the untreated and induced ectoderm are discussed with regard to changes in cell affinity demonstrated in former experiments.