period of growth, its sides become partially coated by the growth of epidermis, and this permanently prevents their proper union by skin-tissue. It is believed that if the _larvæ_ were systematically destroyed in a district, they would soon become extinct, as they are not supposed to travel far. [19] ‘Some Observations on the Œstridæ,’ E. A. Ormerod, Simpkin and Marshall, London, 1884, price 4_d._ A very troublesome injury to the skins of lambs and sheep is the disease known as “cockle,” in which the skin becomes thickly dotted with spots of thickened tissue, which bear some fanciful resemblance in form to a cockleshell. The affection is prevalent during the spring while the wool is thick, and disappears almost immediately on shearing, but little is known of its causes or mode of prevention. Climate and breed have a considerable effect on the quality of hides and skins. As a rule the less highly bred races, and those which are most exposed to the extremes of weather, have the thickest hides, and in most cases highly bred animals have had their meat-producing, or in the case of sheep, their wool-bearing qualities developed at the expense of the characteristics most valued by the tanner. CHAPTER VII. _STRUCTURE AND GROWTH OF SKIN._ Although, at first sight, the skins of different animals appear to have little in common, a closer examination shows that all the Mammalia possess skins which have the same general structure, and thus an anatomical description of the skin of an ox applies almost equally to that of a sheep, goat, or calf, though on account of the difference in texture and thickness the practical uses of these various materials may differ widely. The skins of lizards, alligators, fishes and serpents differ from those of the higher animals, chiefly in having considerable modifications in the epidermis, so that it becomes harder and forms “scales,” and the arrangement of the fibres presents considerable difference. In many fish-skins for instance, the fibres are in successive layers, at right angles to each other and diagonal to the skin, but not interlaced. In its natural condition, the skin is not merely a covering for the animal, but at the same time an organ of sense and of secretion, and hence its structure is somewhat complicated. It consists of two principal layers, the _epidermis_ (_epithelium_, cuticle) and the _corium_ (_derma_, _cutis_ or true skin). These are totally distinct, not only in structure and functions, but in their origin. In the egg of a bird and the _ovum_ of a higher animal, the living germ consists of a single cell, which, as soon as fertilised, begins to multiply by repeated division. The mass of cells thus formed early differentiates into three distinct layers, from the upper of which the epithelium arises, while the true skin, together with the bones and cartilages, is derived from the middle one.