Making Wholemounts 113 



easier to use. Fresh- water Bryozoa should be fixed directly in 4 per cent 

 formaldehyde since they shrink badly in any other fixative. 



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 that 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 overstating and subsequent 

 differentiation in acid alcohol. For most specimens the author prefers 

 Grenadier'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 Delafield. Detailed instructions for the use of this stain on a chicken 

 embryo are given in Example 3. 



Plant Materials. Plant specimens that are to be prepared as whole- 

 mounts often consist of only one or two layers of cells and are therefore 

 easier to stain than zoological specimens. The nuclei may be stained 

 either with safranin, or with an iron hematoxylin 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 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 whole- 

 mounts, 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 of animal material 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 speci- 

 mens because, if they are in specimen tubes, it is almost impossible to 

 transfer them from one to the other without carrying over too much weak 



