16 niSTOLOGIC.\L TECHNIC 



block until the paraffin is slightly softened. This process may be re- 

 peated as often as necessary. 



In addition to the fact that ribbon series can be cut in paraffin, 

 this embedding substance also has the advantage over celloidin that 

 thinner sections can be obtained. On the other hand, celloidin em- 

 bedding produces less shrinkage of the tissues than paraffin embedding 

 with the accompanying heat. 



Frozen Sections. — This method, while more used in pathology 

 where rapid diagnosis is more often important, is still sometimes 

 extremely useful to the histologist, especially as an easy and quick 

 method of testing material. Perfectly fresh tissues may be used or 

 tissues which have been pre\dously fixed. Fixed tissues must have 

 the fixative thoroughly removed before freezing. Of fixatives lo- 

 per-cent. formalin is probably the best and the tissue should remain 

 in the fixative from one to two hours. If time is important the 

 tissue may be boiled for two or three minutes in the formalin 

 solution. 



Ether, rhigolene and carbon dioxid are the most common freezing 

 agents. A special freezing microtome and knife may be used, or a 

 freezing attachment may be used with the ordinary microtome. In 

 ether freezing the ether is vaporized by means of compressed air and 

 the vapor carried to the under side of a metallic plate upon which the 

 block of tissue is placed. In carbon dioxid freezing, which has now 

 largely superseded ether freezing on account of convenience and econ- 

 omy, the commercial cylinder of gas such as is used for charging soda 

 fountains is used. As the liquid carbon dioxid and not the gas is 

 required for freezing, the cyhnder should be hung valve end down, 

 with the valve about the level of the microtome. The carbon dioxid 

 is carried to the metallic disc of the microtome by means of a stout 

 rubber tube and it is advisable to screw a cap with a small hole in it 

 over the valve and to attach a longer handle to the valve lever in order 

 to more perfectly control the flow. The piece of tissue to be frozen 

 should not be over 5 mm. thick and the freezing should be done 

 slowly. A proper hardness of the tissue is important and is of course 

 determined by the amount of carbon dioxid admitted. 



As the frozen section is not supported by any embedding mass, 

 special handling is required, the section being attached to a slide 

 or cover-glass before staining. For this purpose either egg albumen 

 or celloidin may be used. 



