CH.X~\ SERIAL SECTIONING 317 



with absolute alcohol. Work rapidly with only one slide at once. 

 Clear with pure xylene, mount in neutral balsam (§ 386). All 

 nuclei should be blue and all red blood corpuscles, bright eosin red. 

 If one is successful this is a most striking and instructive prepara- 

 tion. Spleen is also very instructive. 



Eosin-methylene blue staining is also excellent for demonstrat- 

 ing mucus (§ 468). 



Do not forget that mercury is liable to be present in sections of 

 tissue fixed with any mercuric fixer. Remove them with iodized 

 alcohol (§ 413). This should be done before the staining. One 

 can tell whether the tissues contain mercury by looking at the 

 unstained sections. The mercury looks black by transmitted light, 

 white by reflected light. The substance is often in the form of 

 delicate black pins. 



MAKING SERIES ; SERIAL SECTIONING 



§ 472. General on Series. — It is coming to be appreciated 

 more and more that in histology as well as in embryology one can 

 only get a complete knowledge of structure by having the entire 

 organ cut in microscopic sections and each section mounted in order. 

 Furthermore it is necessary to have the organ cut in three different 

 planes. In this way one can see every aspect of the structural ele- 

 ments and their arrangement in the organs. 



In single sections one gets only a partial view. For example, 

 how many students have any other idea of a ciliated cell than that 

 it is a cell with triangular outline with a brush of cilia at the broad 

 end. Probably many would be puzzled if they had a top view of 

 the ciliated end ; and the attached end would be even more 

 puzzling. 



It may not be possible for every worker to make serial sections 

 of all the organs in all the three planes, but every one who is work- 

 ing seriously in histology can make all his preparations serial, that 

 is the sections which are mounted can be in serial order, then a puz- 

 zling appearance in one section may be perfectly intelligible in one 

 a little farther along. 



- To get the greatest benefit from serial as indeed also from single 

 sections, the sections should be made in a definite manner, that is, 

 they should be exactly across the long axis of an organ or parallel 

 with the long axis {Transections, and Longisedions) . 



