PREPARATION OF THE SUBSTRATES 169 



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 con- 

 taining the organ should be kept in an ice cupboard. 



Bacteria and other living organisms may be pre- 

 pared 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 



