30 



done. If the tissue has been stained in toto ( 79, 142) the sections may be 

 mounted directly from the clarifier as soon as cut. Any mercuric chlorid precipi- 

 tate (11) that may be present can be dissolved out by means of a solution of iodin 

 in the castor-xylene. The castor oil, however, renders the microtome, knife, hands 

 of the operator, etc., sticky and the method is not so cleanly as the alcohol method. 

 On the other hand, alcohol tends to rust microtome and knife. Clarification is 

 preferred by the writer for serial work in celloidin. 



68. Clarification. Celloidin blocks transferred from the 

 chloroform hardener to an oil mixture such as castor-xylene ( 67) 

 will become quite transparent (clarified) and hardly discernible, so 

 that the tissue is readily seen. Sometimes, however, the celloidin 

 remains white and opaque, due to the presence of moisture, and 

 considerable time is required for its clarification. In such cases the 

 process may be hastened by placing the tissue in the clarifier in a 

 warm place, and changing the clarifier several times. If the block 

 still remains opaque, remove to 95% alcohol for a day for dehydra- 

 tion, pass through chloroform, and into clarifier. In this way the 

 mass may usually be cleared perfectly. Change the clarifier to fresh 

 after the first day or so. The sectioning may be done after a few 

 hours' immersion, although a several days' clarification is preferable.* 



69. Cutting the Sections. There is no marked difference 

 in the sectioning of celloidin blocks preserved in alcohol and those 

 that have been clarified. In the following paragraphs 67% alcohol 

 should in the reading be substituted for clarifier if alcohol was used 

 in the hardening. 



If a paper box was used, after the celloidin is ready for cutting, 

 remove the paper, trim the block as is desired (see below), put some 

 thick celloidin upon the wooden block or other holder and press the 

 base of the celloidin block firmly against it; within two minutes it 

 will be firmly cemented and one may proceed at once to clamp the 

 holder in the microtome and commence cutting. 



For celloidin sectioning, a long drawing cut is necessary in order 

 to obtain thin, perfect sections. The knife should, therefore, be 

 set at an obliquity of 15 to 20 or less, so that half or more of the 



*The imbedded object may remain in the castor-xylene clarifier indefinitely 

 without harm. The celloidin grows somewhat tougher by a prolonged stay in it. 

 After cutting all the sections desired at one time, the imbedded tissue is returned 

 to the clarifier for future sectioning. It should be remembered, however, that 

 pure castor oil is a solvent of celloidin, hence it is necessary to have the container 

 tightly stoppered, otherwise the volatile xylene will evaporate leaving the castor 

 oil behind. 



