The employment of a large number of chemical reagents is one of the unfortunate necessities of modern microtomy. No one can become familiar with the long list of substances employed in microscopical research, nor with the bewildering combinations of fixing agents and staining agents, without realizing the possibility of many of these substances being used without a definite knowledge of their effects. The history of microscopic anatomy is full of instances, in which the results of some pioneer

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... nvestigator have been attacked, on the ground that the results which he obtained were the effects of the reagents employed in the study. In fact, it is even now not uncommon to have the question of the reagent effect brought up, when intricate microscopical studies are being prosecuted. Without doubt, the suspicion that many of the marvellous findings of the cytologists are the products of unknown micro-chem¬ ical changes, is still entertained by many individuals. The controversy between Henle,1 Flemming,2 and others, concerning the nuclear fragmentation ob¬ served in leucocytes and pus corpuscles, is a notable instance of the skepticism relating to the interpreta¬ tion of microscopical findings. Henle maintained that the multi-nuclear condition of leucocytes and pus corpuscles was the result of the acetic acid, or other reagent, used in studying these bodies. This view was strongly opposed by Flemming, who observed multi-nuclear leucocytes in various situa¬ tions in living objects, and who further noted that no change was produced in these cells by killing them with acetic acid, save that the nuclear struc¬ ture became more apparent. Every histologist will remember the discussion which arose when the discovery of the reticulated condition of the protoplasm of the cell and nucleus was announced ; a considerable number of investi¬ gators arraying themselves on the side which main¬ tained that the reticulum was either a faulty optical interpretation, or was the product of the changes induced by the chemical substances employed in the technique. But Flemming and his followers appar¬ ently put the subject at rest, by not only demon¬ strating the nuclear and cytoplasmic reticulum, but also by following the processes of direct and indirect nuclear division in living tissue ; these observations being made by the aid of the most perfect optical appliances. While the employment of reagents is a prime nec¬ essity of microscopical technique, it is nevertheless a fact, that such reagents produce a variety of com¬ pounds with each other and with the substances of the tissue, the exact natures of which are either unknown, or but imperfectly understood. It follows then, that until our knowledge of ultimate microchemical cellular and nuclear reactions be complete, we must regard with a reasonable skepticism, many of the startling microtomical novelties of the day. The open questions in this connection are many. What is the exact nature of that part of the so-called chromatin of the nucleus, which retains certain anilin stains when employed by the "indirect method?"a What is the effect of the fixing medium on this chromatin? What is produced by the combination of the nuclear chromatin and fixing agent, which allows a certain dye to stain precisely, and another one to fail? Many queries of this character con¬ front the microtomist at every turn, and impress him with the profound character of the many unsolved biological problems. It cannot be said, however, that the absorbing subject of the micro-chemistry of staining agents, particularly the aniline, has been entirely neglected; for the work of Ehrlich4 and his students on the reactions of leucocytes, and "mastzellen," indicate the tendency to investigation in this direction.