When first formed the cells are more or less of a spherical shape, and contain a nucleus surrounded by the ultimate photoplasmic substance. Those cells which constitute the core or central portion of the fibre retain to some extent this original globular form and pulpy condition. Surrounding this central portion or medulla, as it has been called (see fig. 3), and forming the main bulk of the fibre, there is a comparatively thick layer of partially flattened cells, which are also elongated in the direction of the length of the fibre, and outside this again there is a thinner stratum which may be compared to the bark of a tree. This outer covering differs materially from the (p. 005) rest of the fibre in its physical structure, but is, probably, nearly identical with it, though possibly not entirely so, in chemical composition. It consists of a series of flattened horny scales, each being probably an aggregation of many cells. The scales, which have been compared to the scales of a fish or to slates on a housetop, overlap each other, the free edges protruding more or less from the fibre, while the lower or covered edges are embedded and held in the inner layer of cells. The free edges always point away from the root of the fibre, just as do the bracts of a fir cone. When viewing a section of a wool fibre there is, of course, no sharp line of division between the three portions above described, but the change from the central spherical cells to the elongated cellular portion, and from these again to the flattened horny scales, is quite gradual, so that the separation into zones, though well marked, is very indefinite in respect of boundaries. The scaly structure of wool is of great importance in regard to what is known as felting property. When woollen fabrics are worked in boiling water, especially in the presence of soap, they shrink in length and breadth, but become thicker in substance, while there is a greater amalgamation of the fibres of the fabric together to form a more compact and dense cloth; this is due to the scaly structure of the wool fibres enabling them to become entangled and closely united together. In the manufacture of felt hats this is a property of very great value. #Variations in Physical Structure.#--Wool fibres vary somewhat amongst themselves; fibres from different breeds of sheep, or even from different parts of the same animal, vary greatly, not only in thickness, length, etc., but also in actual structure. A typical wool fibre, such as may be obtained a good merino or Southdown fleece, will possess the typical structure described above, but frequently the type is departed from to such an extent that the central core of (p. 006) globular cells is entirely absent. Also the serrated character of the outermost layer of cells reaches a much higher state of development in some samples of wool than in others.