524 



METHODS AND FORMULAS 



MS 00-MS 01 



34.4 Cytological methods 



34.5 Bacteriological methods 

 35 Other silver methods 



35.1 Staining solutions 



35.2 Neurological methods 



35.3 Other methods 

 MS 40 OTHER METALS 



41.1 Staining solutions 



41.2 Neurological methods 



41.3 Histological methods 



MS 00 General Observations 



The practice of metal staining, except 

 in neurological techniques, has in late 

 years fallen into some disrepute, largely 

 through the lack of success which has at- 

 tended its use in the hands of the inexperi- 

 enced, or of those not able to observe the 

 very specific precautions which alone can 

 lead to success. The most necessary of 

 these precautions are the utilization of 

 nothing but the purest reagents available, 

 and the maintenance througliout of a con- 

 dition of chemical cleanliness in the glass- 

 ware emi:)loyed. 



It is difficult to justify the retention of 

 the term metallic iviyregnations for this 

 class of microscopic preparation. Many 

 dyes impregnate materials, and there is 

 certainly no justification for the retention 

 of this term to include only those processes 

 which are supposed to result in the deposi- 

 tion, on the surface of the structure to be 

 observed, of a film of metal or metalfic 

 oxides and hydroxides. It is indeed doubt- 

 ful how far a successful metal stain is ever 

 the result of such a deposition. As will be 

 pointed out later, metallic silver is less 

 frequently found in a successful silver 

 stain than is a silver proteinate, nor is it 

 possible always to draw the fine in gold 

 staining between the absorption of gold 

 salts by the tissues and the deposition of 

 very finely divided colloidal gold through- 



out their mass. It seems, therefore, prefer- 

 able to retain the old term metal staining 

 for these reactions. 



Many metal staining techniques should 

 be more widely employed than is at pres- 

 ent the case. This is particularly true of 

 staining with osmium tetroxide or with 

 any of the osmic-chromic fixatives found 

 in Chapter 18. These materials will render 

 the internal organs of a small inverte- 

 brate so sharjjly defined that all after- 

 staining is unnecessary. And no one who 

 has ever examined a jM-operly dehydrated 

 and cleared small crustacean, after fixa- 

 tion in an osmic material, will again be in- 

 clined to try to stain these forms with 

 dyes which yield a more diffuse image. 



At the present time, however, these 

 techniques are almost confined to the cy- 

 tologist searching for Golgi apparatus, or 

 such other lipid materials as may be dem- 

 onstrated by the surface deposition of 

 osmium hj^droxides, or to the neuroanat- 

 omists, the majority of whose discoveries 

 have been made with the aid of silver 

 impregnations. It is to be hoped, however, 

 that the classification of stains which fol- 

 lows will encourage more biologists to en- 

 deavor to utihze these excellent and rapid 

 techniques more generally than is at pres- 

 ent the case. 



MS 01 CLASSIFICATION OF METHODS AND FORMLTLAS 



Two bioad (H)nsidenitions necessitate 

 the employment of a classification of metal 

 stains different from that used for the dye 

 stains in Chapters 20 and 21 . First, none of 

 the metal stains aio of such general appli- 

 cation tliat a division of "stains of general 

 application" is possible. Second, only 



three metals — in contrast to several hun- 

 dred dyes— are commonly used, so that 

 "osmium," "gold," "silver," and "other 

 metals" form convenient titles for pri- 

 mary divisions. 



Many metal-staining methods require 

 that the tissues shall have some special 



