Such a preparation is shown in Plate IX. Fig. 39, but no drawing can give an idea of its wonderful beauty and interest. The cilia will continue to move for a long time after removal from the shell. * * * * * Bone in its various stages is figured on Plate X. Fig. 9 is a good example of human bone, and is a thin transverse section taken from the thigh. When cut across, bone exhibits a whitish structure filled with little dottings that become more numerous towards the centre, and are almost invisible towards the circumference. In the centre of the bone there is a cavity, which contains marrow in the mammalia and air in the birds. When placed under a microscope, bone presents the appearance shown in the illustration. The large aperture in the centre is one of innumerable tubes that run along the bone, and serve to allow a passage to the vessels which convey blood from one part of the bone to another. They are technically called Haversian canals, and if a longitudinal section be made they will be found running tolerably parallel, and communicating freely with each other. Around each Haversian canal may be seen a number of little black spots with lines radiating in all directions, and looking something like flattened insects. These are termed bone-cells or “lacúnæ,” and the little black lines are called “canalículi.” In the living state they contain cells which are concerned in the growth of the bone, and these may be made evident by softening fresh bone with acid, cutting sections of it, and staining. When viewed by transmitted light the lacunæ and canaliculi are black; but when seen by dark-field illumination the Haversian canals become black, and the lacunæ are white. As these canaliculi exist equally in every direction, it is impossible to make a section of bone without cutting myriads of them across; and when a high power is employed they look like little dots scattered over the surface. A very pretty object can be made of the bone taken from a young animal which has been fed with madder, as the colour gets into the bone and settles chiefly round the Haversian canal. A young pig is a very good subject, so is a rabbit. Fig. 16 is a similar section cut from the leg-bone of an ostrich. The development of bone is beautifully shown in Fig. 30, a delicate slice taken from a pig’s rib. Above may be seen the gristle or cartilage, with the numerous rows of cells; below is the formed bone, with one of the Haversian canals and its contents; while between the two may be seen the cartilage-cells gathering together and arranging themselves into form. The cartilage-cells are well shown in Fig. 28, which is a portion of the cup which had contained the eye of a haddock.