152 Invertebrate Zoology. 



the tissue and leave other parts unaffected. Selective stains 

 are for this reason spoken of as " differential stains." Cer- 

 tain tissues will absorb a certain color in a certain fixed way 

 and may be quite unaffected by another stain. 



The stains more commonly used are made from carmine, 

 cochineal, hsematoxylin, or from some of the numerous ani- 

 line dyes. f 



Stains are also either aqueous or alcoholic. Alcoholic 

 specimens may be placed directly into alcoholic stain, but 

 should be first washed with water before being placed in 

 aqueous stain. The length of time that an object requires 

 for proper staining depends on the object, the way it has 

 been hardened, the length of time it has been preserved, and 

 the stain that is adopted. 



The objects are taken from the alcoholic stains, washed 

 in acidulated alcohol for a short time, and then placed, until 

 wanted, in 70-90% alcohol. If an aqueous stain has been 

 used, the excess of stain is washed out with water, often acid- 

 ulated, and then the object is placed successively in 50, 70, 

 and 90% alcohol. 



H. 



MOUNTING. 



MANY small organisms, and portions of larger, may be 

 mounted on glass slides and retained, if properly prepared, 

 indefinitely. The mounting medium most commonly used is 

 Canada balsam. The order of procedure is as follows : 



The object, stained or unstained, is carried up through 

 the grades of alcohol, and is finally placed in absolute alco- ' 

 hoi, that the last trace of water may be withdrawn from it 

 This is the process of dehydrating. 



