CH. Xr] SERIAL MICROSCOPIC SECTIONS 401 



well. If the end of the slide is varnished, one can write on it as 

 well as on paper. When the ink is dry it should be coated with thin 

 xylene balsam or with any good varnish like valspar i part, xylene 

 9 parts. It is also important to write the number of the slide with 

 a writing diamond. The double marking is desirable because with 

 wet slides the diamond number is hard to see, while the ink marks 

 are clearly visible. One is not so liable to wipe off the sections if the 

 ink mark is present. 



FIXING AND STAINING FOR SERIES 



652. Fixing. The two most used fixers for embryos are Zenker's 

 fluid and formaldehyde (568, 592). For those unskilled in micro- 

 scopic technic, or for one who is exceedingly busy, the best results 

 are obtained by putting the embryos in formaldehyde (10 parts of 

 formalin, the formalin of the pharmacy, and 90 parts water answers 

 well). If there is plenty of this the embryos are likely to be well pre- 

 served even though they are left in the membranes, and that is far 

 the best way for small embryos. 



653. Fastening the sections to the slide. For all serial work 

 it is especially desirable to fasten the sections to the slide with collo- 

 dion ( 622). This should always be done unless some stain like car- 

 mine is to be used on the slide after the sections are fastened. With 

 thin sections, if one is careful enough, an entire series can be carried 

 through without losing a section, but with thick sections (15^ and 

 thicker) some are almost sure to separate from the slide if not fas- 

 tened by collodion. 



654. Removal of mercuric chlorid from sections. It should be 

 remembered that if a fixer containing mercuric chlorid is used the 

 sections are almost sure to contain mercury. By transmitted light 

 the mercury appears dark. Often the appearance is as if a multitude 

 of delicate black pins were in the section. Sometimes the mercury 

 is in rounded masses. This should be removed by putting the slides 

 of sections into alcoholic iodin ( 576). After half an hour or an hour 

 wash off the iodized alcohol with pure 95 % alcohol and the sections 

 are ready for staining. 



If the embryo was stained in ioto and contains mercury, the sec- 



