immersion in water or it will be tough and will shrink. Handling of the celloidin-sheets with the bare hands is not advisable because of the large number of epithelial cells adhering to the celloidin. The sheets are easily changed from one solution to another by catching them with forceps; the use of a glass-plate to transfer them is not necessary. When it is desired to preserve sections for future staining the celloidin sheet containing the paraffin-sections can be kept in 80 per cent alcohol indefinitely. NOTE:—If the glass-plate is numbered with a blue wax-pencil after the paraffin sections are floated on, the marking will be transferred to the celloidin sheet, and the latter will retain the marking through all solutions. II. STAINING AND DIFFERENTIATION. Staining is necessary to bring out clearly the constituent elements of the tissues and their relations with each other, and for the demonstration of histologic structures or chemical substances that would otherwise be nearly or wholly invisible. The technique of staining depends upon the fact that stains or dyes possess certain affinities for the tissue-elements or for certain simple or complex substances present in the tissues (_microchemic reactions_). These affinities vary greatly with the dye. Some dyes have an affinity only for single constituents of the tissue (_elective_ or _specific stains_); others have an especial affinity for the nucleus (_nuclear stains_), others stain all the tissue-constituents diffusely (_diffuse_ or _protoplasmic stains_). There are but few pure elective stains for single tissue-elements; the majority of stains will stain more than one of the tissue elements, but may show an especial affinity for certain ones. As a result of these variations in the affinities of dyes for the constituents of the tissues it becomes possible to manipulate the dyes or to combine them in such a way that a specific differentiation of many tissue-elements is possible through the use of _different methods of staining_. These methods are based in part upon the use of different mordants, the employment of several stains in combination or in succession, the mixture of stains to form a new staining compound, the phenomenon of _metachromasia_, the differentiation of certain tissue-elements by the removal of the stain from the structures for which it possesses a weaker affinity, and by the employment of different microchemic reactions. The two most commonly employed methods are the _progressive_, in which the stain is allowed to act until the affinities of certain tissue-elements have been satisfied when the staining process is interrupted; and the _regressive_, in which the tissue is over-stained, and the dye withdrawn from the tissue-elements for which it possesses the weakest affinities leaving the other elements stained. This latter process is usually called “_differentiation_,” and the chief substances used for such differentiating are dilute