44 MATERIALS AND PROCESSES OF SLIDE-MAKING 



Arthropods. Wholemounts of most small arthropods are better made in 

 gum media. 



Choice of a Stain. Whatever method of narcotization and fixation has been 

 employed, the specimens, which are to be mounted, are washed free of fixative 

 and accumulated either in water or 70 per cent alcohol. 



Small Invertebrates and Invertebrate Larvae. These are best stained 

 in carmine by the indirect process; that is, by overstaining and subsequent 

 differentiation in acid alcohol. For most specimens the author prefers Gren- 

 acher's alcoholic borax carmine. 



Larger Invertebrate Specimens. Larger specimens are better stained by 

 the direct process; that is, exposed for a considerable length of time to a very 

 weak solution of stain and not differentiated. 



Vertebrate Embryos. These seem to stain more satisfactorily in hema- 

 toxylin than in carmine solutions. The author's preference is the formula of 

 Carazzi. Detailed instructions for the use of this stain on a chicken embryo are 

 given on page 89. 



Plant Materials. Plant specimens, since they often consist of only one or, 

 at the most, two layers of cells are easier to stain than are zoological speci- 

 mens. The nuclei may be stained either with safranin or with an iron hema- 

 toxylin technique which in zoological procedures is rigorously confined to 

 sections. A contrasting plasma stain may be used after the nuclei have been 

 well differentiated. 



Dehydration. The specimens, plant or animal, stained or unstained, are 

 accumulated either in distilled water or in 70 per cent alcohol according to the 

 treatment which they have had. It is necessary to remove the water from them 

 before they can be mounted in any resinous medium. Ethyl alcohol is widely 

 used as a dehydrant, and, at least in the preparation of wholemounts, only its 

 unavailability should make any substitute necessary. Where substitution is 

 necessary, acetone or methyl alcohol, in that order of preference, may be used. 

 Both have the disadvantage of being more volatile than ethyl alcohol and, 

 therefore, requiring more care in handling. 



Dehydration is carried out by soaking the specimen in gradually increasing 

 strengths of alcohol; it is conventional to employ 30 per cent, 50 per cent, 

 70 per cent, 90 per cent, 95 per cent, and absolute alcohol. The author prefers 

 to omit from this series, unless the object is very delicate, both the 30 per cent 

 and the 50 per cent alcohol, thus starting with direct transfer from water to 

 70 per cent alcohol. The only difficulty likely to be met in dehydration is in 

 the handling of small specimens because, if they are in specimen tubes, it is 

 almost impossible to transfer them from one to the other without carrying 



