The Microscope. 335 



as given in § 13. When the slide is full, pour over the sections 95 

 per cent, alcohol, and then dissolve out the celloidine with a few drops 

 of ether and absolute alcohol. Let these nearly evaporate, wash again 

 in 80 per cent, alcohol, clear and raount as in§ 13. If the sections 

 have not been stained, after treating with 80 per cent, alcohol, wash 

 in water, stain, dehydrate in alcohol, clear and mount, according to 

 the above method. 



In order that sections may be arranged with exactness on the 

 glass slips, it is well to have a black outline of a slide, with cover- 

 glass, on a piece of cardboard, or the very convenient centering card, 

 mentioned by Prof. Gage in Vol. VI, p. 266, of this journal, may be 

 used. In order to study the embryos prepared as above directed, it 

 is necessary that the sections should be made in various directions 

 through the specimen. Thus, in order to intelligently comprehend 

 what we see in a transverse section (transection) in one embryo, we 

 must have sections taken lengthwise, from behind forward, (dorso- 

 ventral longisections) from another embryo of the saa?e age as the 

 first, and sections in the long axis of the body, from right to left 

 (dextro- sinistral longisections), from still another specimen. 



SECTION V. 



EMBRYOS AS TRANSPARENT OBJECTS. 



§ 15. From the salt solution, the specimen must be pinned out 

 in a little of the osmic acid solution, in which it is kept until it 

 becomes of a light brown color. It is then transferred to water, and 

 washed for some time, to remove the osmic acid. The embryo must 

 then be placed in 70 per cent, alcohol, then in absolute alcohol, which 

 latter must be changed once, and finally in clove oil to clear up. 

 This accomplished, the embryo is ready for mounting. 



Having cleaned a slide, place a drop of Canada balsam on its 

 center, and lay the embryo on this. All clove oil is now run ofP and 

 wiped away, and another drop of Canada balsam allowed to fall 

 upon the specimen. The corners of the cover-glass are now tipped 

 with wax, to slightly raise it from the specimen, and the cover then 

 adjusted. 



On either side a hair of spun glass must b6 run beneath the 

 cover glass, to keep this permanently up off the specimen. This 

 glass-hair is quickly made by heating a bit of glass rod or tubing in 

 the lamp-flame and then drawing it out as fine as possible, while hot. 



When the balsam is dry, the wax on the cover-glass may be 

 removed. Embryos may be treated in this manner up to the forty- 

 eighth hour, but after that they are rather too large to make good 

 specimens. 



