_The Fat-body._ Adhering to the inner face of the abdominal wall is a cellular mass, which forms an irregular sheet of dense white appearance. This is the fat-body. Its component cells are polygonal, and crowded together. When young they exhibit nuclei and vacuolated protoplasm, but as they get older the nuclei disappear, the cell-boundaries become indistinct, and a fluid, loaded with minute refractive granules,[98] takes the place of the living protoplasm. Rhombohedral or hexagonal crystals, containing uric acid, form in the cells and become plentiful in old tissue. The salt (probably urate of soda) is formed by the waste of the proteids of the body. What becomes of it in the end we do not know for certain, but conjecture that it escapes by the blood which bathes the perivisceral cavity, that it is taken up again by the Malpighian tubules, and is finally discharged into the intestine. The old gorged cells probably burst from time to time, and the infrequency of small cells among them renders it probable that rejuvenescence takes place, the burst cells passing through a resting-stage, accompanied by renewal of their nuclei, and then repeating the cycle of change. [98] The granules are not shown in the figure, having been removed in the preparation of the tissue for microscopic examination. The segmental tubes forming the Wolffian body of Vertebrates have at first no outlet, and embryologists have hesitated to regard this phase of development as the permanent condition of any ancestral form.[99] It is, therefore, of interest to find in the fat-body of the Cockroach an example of a solid, mesoblastic, excretory organ, functional throughout life, but without efferent duct. [99] Balfour, Embryology, Vol. II., p. 603. [Illustration: Fig. 38.--Fat-body of Cockroach, cleared with turpentine. _A_, young tissue, with distinct cell-boundaries and nuclei, a few cells towards the centre with dead contents; _B_, older ditto, loaded with urates, the cell-walls much broken down, and the nuclei gone; _tr_, tracheal tubes. × 250.] The fat-body is eminently a metabolic tissue, the seat of active chemical change in the materials brought by the blood. Its respiratory needs are attested by the abundant air-tubes which spread through it in all directions. The considerable bulk of the fat-body in the adult Cockroach points to the unusual duration of the perfect Insect. It is usually copious in full-fed larvæ, but becomes used up in the pupa-stage. Extensions of the fat-body surround the nervous chain, the reproductive organs and other viscera. Sheets of the same substance lie in the pericardial sinus on each side of the heart. _The Cœlom._