Reagents 31 



be more than -f 6 inch from the surface. Even then, it must be remem- 

 bered that a waxy or cutinized or suberized surface presents an almost 

 impassable barrier to the chromic series. 



Some objects, although small, cause trouble in various ways. 

 Many buds are hairy and will not sink; if such things are dipped 

 quickly in strong alcohol, they will usually sink. If rather large 

 air bubbles prevent the material from sinking, as in case of peri- 

 chaetical leaves of some mosses and involucral leaves of liverworts, a 

 little dissection or a careful snip with the scissors will obviate the 

 difficulty. If an air-pump is available some bubbles are easily 

 removed, but air bubbles in cells may resist even the air-pump. 

 Heating followed by rapid cooling is recommended by Pfeiffer and 

 Wellheim for removing air, but, for cytological work, the remedy is 

 worse than the bubbles. 



It is often asked whether fixing agents really preserve the actual 

 structure of cell contents. It must be admitted that some things- 

 notably the liquid albuminoids are much modified in appearance, 

 but the most competent observers are now inclined to believe that 

 such delicate objects as chromosomes, centrosomes, the achromatic 

 figure, and even the structure of protoplasm, can be studied with 

 confidence from material which has been fixed, imbedded, and 

 stained. Extensive investigations upon various objects in the living 

 condition have strengthened this confidence. 



It is certain that we have not yet found the ideal fixing agent 

 for cell contents. Such an agent must not be a solvent of any of the 

 cell contents, must penetrate rapidly, must preserve structures 

 perfectly, and must harden so thoroughly that every detail shall 

 remain unchanged during the subsequent processes of dehydrating, 

 clearing, imbedding, sectioning, and staining. 



DEHYDRATING AGENTS 



Objects which are to be imbedded in paraffin or celloidin, and 

 also all other objects which are to be mounted in balsam, must be 

 dehydrated, i.e., they must be freed from water. The slightest 

 trace of water is ruinous. Alcohol is used almost exclusively for 

 dehydrating. The process must be gradual. If material has been 



