The choice of methods will depend upon the source and nature of the material, the object of the examination, the time-element and the degree of responsibility involved. The cellular elements of all pathologic fluids, secretions and excretions should be examined in the fresh state as well as in fixed preparations. For the demonstration of various chemical and morphologic features that are lost or altered by processes of fixation and imbedding, and when a rapid diagnosis is required, the examination of the material in its fresh state or by the freezing method is indicated. When the freezing method cannot be employed because of the changes in cells and tissue produced by it, when very thin or serial sections are desired, when a rapid diagnosis is not required, and when a very careful and minute study is desired, with the application of various staining methods, then the material should be fixed, hardened and imbedded and cut upon a microtome. Whatever method is chosen, it must be borne in mind, particularly in practical diagnostic work, that the portions chosen for microscopic examination must represent the characteristic anatomic structures of the tissue or organ, that living tissue be included, that the pathologic condition be represented both in its fully-developed state and at the transition-border between it and the healthy tissue, and that when sections are cut the block or tissue must be so oriented as to give the most comprehensive view of the tissue and the pathologic process. To accomplish this fully it is often necessary to make a number of blocks representing different areas of the material, and to cut these in different planes. CHAPTER XVII. THE LABORATORY OUTFIT.