144" 



into the stain; the layer of collodium containing the sec- 

 tions then separates from the glass plate, and it can be 

 treated as a single section. 



Paraffin Embedding. 



Undoubtedly the paraffin method of embedding is of 

 all methods the most satisfactory for general work in his- 

 tology, embryology, and pathology. Only very few tissues 

 met with, can not to advantage be embedded in this way. 

 Exceedingly thin sections can be cut when the tissues are 

 properly embedded in paraffin, and as the paraffin is 

 always removed from sections before staining, it has none 

 of the disadvantages of the celloidin method. The tissues 

 after thorough dehydration are placed for a longer or 

 shorter time into a fluid which mixes readily with alcohol 

 on the one hand and is a good solvent for paraffin on the 

 other hand. The method used in this laboratory is 

 briefly as follows : 



(1) The tissues are thoroughly dehydrated in absolute 

 alcohol. This takes from 2 to 24 hours, depending on the 

 size of the pieces, the density of the tissue and the exteixt 

 of dehydration before placing in absolute alcohol. 



(2) The tissues are then transferred to toluol or xylol 

 (chloroform, turpentine, or oil of origanum may be used) 

 for 4 to 24 hours. Toluol is preferred as it mixes very 

 readily with absolute alcohol and is one of the very best 

 solvents of paraffin. In case the tissues are not thoroughly 

 dehydrated before placing in the toluol or xylol, these 

 fluids become milky and the tissue should then be again 

 transferred to absolute alcohol until the dehydration is 

 complete. 



(3) The tissues are then placed into melted soft paraf- 

 fin in which they remain for 1 to 12 hours, again depend- 

 ing on the size of the piece of tissue to be embedded, its 

 density, etc. Soft paraffin has a melting point of about 38 

 to 42 C. The cup, jar, or bottle containing this paraffin 

 is kept in a warm oven, the temperature of which can be 

 regulated; it should not exceed 50 to 52 C. if delicate 

 tissues, embryos, etc., are to be embedded, it is well to add 

 from time to time a few small pieces of unmelted soft 



