2 THE ELEMENTS OF VEGETABLE HISTOLOGY 



which infiltration with a supporting medium is unnec- 

 essary. 



INFILTRATION METHOD 



This method of treating materials for sectioning is 

 used in thf preparation of specimens of animal tissues 

 and the finer types of vegetable sections, especially 

 those usea to illustrate the protoplasmic contents and 

 the parts of the living cell. Ii brief, the process con- 

 sists in killing the tissue without' disturbance of the 

 cell contents and involves a replacement of the water in 

 the cells by a supporting medium. The necessary 

 operations include fixation, dehydration, clearing, infil- 

 tration, embedding, cutting and staining. 



Fixation.— The object of this process is to abruptly 

 terminate the life of the cell, without causing too great 

 a disturbance of the protoplasmic contents, and to 

 harden materials of very soft texture. By this means 

 we secure specimens showing the protoplasmic elements 

 of the cell. The fixing agents in common use include 

 formalin, picric acid, chromic acid, chromates, osmic 

 acid and combinations of mercuric chloride with other 

 salts. Formulae for several fixing solutions will be 

 found in the Appendix. Specimens should not exceed 

 8 mm. in size, else the fixing fluid will not rapidly 

 penetrate the material. The time required for thor- 

 ough fixing depends upon the texture of the material, 

 the fixing fluid employed and the size of the specimen. 

 After fixation the specimen must be thoroughly washed 

 in water unless otherwise directed. If colored fixing 

 fluids are used, it is well to continue washing until the 

 wash water is free of color. If stained specimens are 

 desired, the staining may be performed after washing 



