320 SERIAL SECTIONS OF EMBRYOS [CH. X 



be well preserved even though they are left in the membranes, and 

 that is far the best way for small embryos. 



§ 476. Fastening the Sections to the Slide. — For all serial 

 work it is especially desirable to fasten the sections to the slide with 

 collodion (§ 450). This should always be done unless some stain 

 like carmin 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,/u and thicker) some are almost sure to separate from the slide. 



§ 477. 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 trans- 

 mitted 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 (§ 413). 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 toto and contains mercury, the 

 sections should be passed from the deparaffining xylene to the 

 iodized alcohol (§ 413). After half an hour or more the slides are 

 passed through pure 95% alcohol, and back to the xylene or to 

 carbol-xylene. Then they can be mounted in balsam. 



§ 478. Staining for Series.— There is a great advantage in 

 point of time and safety in staining the entire embryo in some good 

 stain like borax carmin (§ 388). Carmin is a very permanent 

 stain also. For bringing out special structural details the sections 

 are stained on the slide as described in §461-471. The slide 

 baskets are almost a necessity for serial work (Fig. 244, 251), as the 

 slides are handled individually only twice, ( 1 ) when they are spread 

 and dried and put into the baskets, and (2) after all the processes 

 are complete and the sections are to be mounted in balsam. 



The sections are mounted in balsam directly from the depara- 

 ffining xylene. No alcohol is used unless it is necessary to remove 

 crystals of mercuric chlorid (§ 477). 



SERIAL SECTIONS OF EMBRYOS 



§ 479. Serial Sectioning Embryos and Minute Animals.— 



