_Fig._ 1. _a._ Spheroidal epithelium cells, filled with central nuclei and granular matter; _b._ mucous membrane of stomach, showing cells, with open mouths of tubes at the bottom of each, magnified 50 diameters--2. _a._ Diagram of a portion of the involuted mucous membrane, showing continuation of its elements in the follicles and villi, with a nerve entering the submucous tissue. The upper surface of one villus is covered with cylindrical epithelium; the other denuded, and with dark line of basement membrane running around it; _b._ epithelium cells, separated and magnified 200 diameters, a central nucleus, with a nucleolus, seen in centre; _c._ pavement epithelium cells, from the mucous membrane of bronchial or air tubes with nuclei, and nucleoli in some; _d._ vibratile or ciliated epithelium, nuclei visible, and cilia at the upper free surface, magnified 200 diameters--3. _a._ is one of the tubular follicles from a pig’s stomach, cut obliquely to display upper part of cavity, and the cylindrical epithelium forming its walls, a few cells detached; _b._ shows a section of a lymphatic, with capillary blood-vessels, distributed beneath the mucous surfaces--4. Cells of adipose tissue, or fat, magnified 100 diameters--5. a single fat-cell separated, and magnified 250 diameters--6. A capillary of blood-vessels distributed through tissue--7. Section of the Tendo-Achillis as it joins the cartilage, showing stellate cells of tendon, seen to be gradually coalescing to form round or oval cells of cartilage--8. A vertical section of cartilage, with clusters of cells arranged in columns previous to their conversion into bone--9. A small transverse section of the same, showing the gradual change of the cartilage cells at _a._ into the true bone cells, _lacunæ_, at _b._ with characteristic canaliculi--10. A stellate nerve corpuscle, with tubular processes issuing forth, at _a._ filled with corpuscles containing black pigment, above which is a corpuscle the nucleus of which is seen to have nucleoli; at _b._ a corpuscle enclosed within sheath, and filled with granular matter taken from the root of a spinal nerve--11. The continuity of muscle, the upper portion, with connective tissue of the lower portion, from the tongue of a lamb--12. Branched muscle, ending in stellate connective cells, from the upper lip of the rat--13. Choroidal black pigment-cells from the human eye. PLATE XX.--_Page_ 658. BONE STRUCTURE.