It must be emphasized here that the process of freezing is an active one, and alters the relation of cell-structures. With many fresh tissues the changes resulting from the freezing are so great that no diagnosis can be made. It seems necessary here to warn against the routine employment of the rapid method of freezing and staining fresh tissues in the diagnosis of material obtained by surgical operation. It has become a fad with some surgeons to make a pathologic diagnosis by the freezing method while the patient is on the table. Consequently, as the result of diagnoses made by the rapid freezing and staining method, many mistakes are made, even by supposed experts in this line. Particularly in the diagnosis of sarcoma is it easy to make mistakes because of the altered aspect of the cells caused by freezing. Normal lymphnodes, tonsils and inflammatory infiltrations may look like spindle-cell sarcoma in the sections prepared by the rapid freezing and staining method; and the exact nature of many other pathologic conditions cannot be accurately determined from such sections. On the other hand, a certain number of pathologic conditions can always be recognized in sections obtained in this way, and this fact justifies the employment of the method when properly controlled. _In all cases in which the pathologic condition is not clearly evident in sections obtained by the rapid freezing and staining method no diagnosis should be given._ In such cases the tissue should be fixed and then cut upon the freezing-microtome, or imbedded in celloidin or paraffin and then cut. Even when the patient is upon the table the tissue removed can be put into a 10 per cent formol solution for a few minutes and then frozen directly in gum without washing out the formalin. The longer the time that can be used for this short preliminary fixation the better the sections will be and the less the production of artefacts by the freezing. The process of fixation can be hastened by warming the fixing-fluid. I advise this short fixation before freezing in all cases of operative diagnostic work when the diagnosis is wanted as soon as possible. For all other work with the freezing microtome, when the question of time is not so important, fixation with formol for 12-24 hours should be carried out. This combination of formol-fixation and the freezing method permits the early diagnosis of autopsy and operation material, makes possible the demonstration of fat and other substances altered or dissolved out by the imbedding methods, and is a convenient way of selecting tissues, requiring more complicated staining processes. (See also Page 239.) The further treatment of sections of fresh tissue obtained by freezing will be found in the second section of this chapter.