The Microscope. 145 



Kleinenberg or chromic acid solution,* the embryo transferred to it 

 and the edges of the filmy membrane carefully drawn out and 

 pinned down to the wax, so that the embryonic disk may harden 

 without wrinkling or distortion. Any kind of pins may be used for 

 this purpose, but the long German insect-pins are the easiest to 

 handle, and are not so readily afPected by the acid. 



A. Kleinenberg's Picric Acid Solution. — The embryo should 

 be left in this undisturbed to harden, from 5 to 24 hours, according 

 to size. 



When this is accomplished, prepare a glass jar — a half-pint 

 fruit jar or a tumbler will do — with 70 per cent, alcohol, in which a 

 piece of cotton floats just below the surface. Upon this cotton place 

 the embryo, and allow it to remain until the alcohol has withdrawn 

 the picric acid, and the specimen has become whitened. It will 

 sometimes be found necessary to change the alcohol once or twice 

 before this is accomplished. When the embryo is quite bleached it 

 may either be stained immediately, or placed at once in 95 per cent 

 alcohol for indefinite keeping. 



B. Chromic Acid Solution. — The method of procedure is the 

 same as that just given, except that the hardened specimen is trans- 

 ferred to a jar of water, instead of alcohol. When washed free from 

 the acid — which requires some little time — the embryo is placed 

 first in 70 per cent, alcohol, then stained — or placed in 95 per cent, 

 alcohol for keeping. .^ ■ y^ 



P 



A COURSE IN ANIMAL HISTOLOGY. 



FRANK W. BROWN, M. D. 



SECOND PAPER. 



CONCLUDED. 



RACTICAL WORK.— (A.) With a clean knife-blade scrape the 

 surface of the tongue or the inside of the cheek sharply. 

 Place a small quantity of the scraping, not larger than a pin's head, 

 on the slide, and add to it a drop of the normal salt solution. Mix 

 thoroughly by means of a needle, adjust the cover-glass and examine 

 with a ^ or 1^ inch objective. In the field will be seen a number of 

 cells packed together in masses. Avoid these and look more to the 

 open spaces for isolated cells. As they are unstained, the untrained 



* After some little experience with both of these hardening agents, I have come to 

 greatly prefer the picric acid solution, for the reason that the specimen is rendered less 

 brittle, and takes the stain much better than when the chromic acid i.s used. For some 

 embryos the Kleinenberg solution is too strong and must be diluted with water. The 

 formula as given does well, however, with chick embryos. 



