86 The Preparation of Microscope Slides 



but is also much the best method of staining the nuclei in blocks of tissue 

 before sectioning if this technique is to be employed. 



Grenacher's alcoholic borax carmine is not very satisfactory when 

 diluted and used by the indirect method. For this purpose it is recom- 

 mended that the following be employed: 



Mayer's Carmalum: 



Staining solution 



Boil together, for 1 hr, 2 g of carmine and 5 g of potassium alum in 

 100 ml of water. Cool and filter. 



This solution may be used exactly as Grenacher's alcoholic borax 

 carmine for wholemounts or blocks of tissue but it is less satisfactory for 

 this purpose. It is best used after great dilution for direct staining of rela- 

 tively large invertebrates. 



Method to Be Employed Is as Follows: 



1. Accumulate the objects in distilled water. 



2. Dilute the stain with 5 per cent potassium alum to the required con- 

 centration. The larger the object to be stained, the lower the concentra- 

 tion of the staining solution should be. For a relatively small and thin 

 object, such as the prothallium of a fern, a dilution of about 10 to 1 is 

 satisfactory. For a large object, such as a liver fluke, the stain should be 

 diluted until it is only faintly pink. 



3. Transfer the object to the stain and leave it until examination shows 

 the nuclei or internal structures to have been stained a fairly dark red, 

 while the other portions are stained only pink. This will take about 6 

 hr with a fern prothallium in the dilution described or about 3 weeks for 

 a liver fluke at the low concentration. 



4. Wash the object in running water until all alum has been removed 

 and then mount in the ordinary manner. 



ORCEIN 



Many lichens contain a crude pigment which, even as late as the early 

 nineteenth century, was used to dye textiles. An oxidation product of 

 one of the ingredients of this pigment is used by cytologists, under the 

 name "orcein," to stain chromosomes in squash preparations. It has no 

 other use. The following technique is the best known: 



LaCour's Aceto-Orcein 



Water 20 to 50 ml 



Acetic acid 70 to 30 ml 

 Orcein 1 g 



