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 preserve indefinitely, and it only becomes useless again by being contaminated. There are various contingencies that may spoil a perfect organ. In the first place, it must be taken out of the bottle only by means of sterilized forceps. None of the sample taken should be put back into the bottle if it has been exposed to the risk of infection, or been left lying about, and so on. The bottle must be kept filled with toluol, otherwise part of the tissue may adhere to the neck of the bottle. If such a piece protrudes from the level of the toluol it decays, and finally drops down on to the rest of the tissue. The bottle containing the organ should be kept in an ice cupboard.

Bacteria and other living organisms may be prepared exactly in the same way as tissues. Boiling is also resorted to in these cases; and the same rules hold good. It is obvious that organs can be separated into their tissues. The more special the problems to be dealt with, the more does one limit oneself to a very definite tissue.

All organs which are very dense in structure, and which become hard when boiled, require special treatment. Carcinomas, myomas, &c., may appear snow-white and still contain blood, so that in these cases the pieces have to be cut into very minute particles in order to prevent mistakes.

Every organ must be standardized. Placenta is only useful so long as it is not decomposed by the