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Investigations of the domain structure (DS) of nickel single crystals of cylindrical and prismatic form with [100] and [110] axes are reported. Using the Bitter technique we observe the colloid pattern on all faces around the crystal axis. The total DS is divided into the basic DS and the closure DS. The basic DS of the [110] crystals is a simple plate structure. The plates (approximate width 0.05 mm) are normal to the crystal axis and each consist of a single elementary domain. Neighboring plates are separated by 71° Bloch walls (forH_{e} = 0). The basic DS of the [100] crystals also consists of plates (0.13 mm) normal to the axis, but now each plate contains several elementary domains; we call this a composed plate structure. In axial direction adjacent domains are separated by 109° Bloch walls. The basic DS is essentially the same in prismatic and cylindrical crystals. The closure DS of the [110] crystal forms a pattern of lines normal to the axis. The most remarkable result is, that the number of lines increases with increasing field; the closure DS varies not only quantitatively, but qualitatively with the field. We succeed in exploring many details of the secondary structure, so that we can draw quite a complete picture of the DS of these nickel crystals.

RECENT investigations have again indicated that the thymus produces a hormone which stimulates the lymphoid tissues of the body1. In 1956, Metcalf indicated that the plasma of patients with lymphatic leukaemia contained a lymphocytosis-stimulatmg factor2. Metcalf's investigations on the extract from thymic tissue, as well as the work of other research workers, link the thymus with leukaemia3,4.