VL DETERMINATION OF AMOUNT OF 

 A BIOLOGICAL SAMPLE 



A major problem in histo- and cytochemistry is the quantitative 

 definition of the samples of biological material which are to be con- 

 sidered. Weight, volume, numbers of cells, and nitrogen or nucleic 

 acid content have been variously employed for this purpose, and 

 both advantages and disadvantages are to be found in each case. The 

 actual choice of a reference quantity will depend on the particular 

 problem to be considered. 



Weight. Wet- and dry-weight measurements may be made with 

 commercial microbalances including torsion balances such as those 

 manufactured by Roller-Smith Co., which are sensitive down to 

 about 2 fjLg. Quartz fiber balances of greater sensitivity are described 

 on page 189. 



It is advantageous in some instances to employ the so-called "re- 

 duced weight" which is the weight of the sample minus the weight 

 of an equal volume of water. The feature of this value is that it is 

 independent of the water content of the sample. "Reduced weight" 

 can be determined rather simply in the gradient tube apparatus by 

 placing the sample in a drop of water and measuring the density of 

 the drop. From the density, the volume of the drop, and the density 

 of water, the "reduced weight" can be calculated (page 421). 



Volume. When the sample, such as an Amoeba profeus, is of a 

 nature and size that permits it to be drawn into a capillary tube of 

 known diameter, the volume can be calculated after the length of the 

 sample in the tube is measured. However, this procedure cannot be 

 used if the sample has an irregular shape which does not allow it to 

 completely fill the lumen of the capillary over the entire length of 

 the body or if it is so sticky that it adheres to the wall of the capil- 

 lary and thus interferes with the manipulation or becomes damaged. 

 To determine volumes in these cases, Holter (1945) developed a 

 colorimetric method which he applied to measurements of the amoe- 

 ba, Chaos chaos. The details of the procedure are given on page 432. 



The volume of microtome sections of tissue is regulated by cut- 



423 



