8 INTRODUCTORY. 



itself over the surface of a slide, but lias a tendency to form 

 very convex drops. This property also makes it frequently a 

 very convenient medium for making minute dissections in. 



5. Following Paul Mayer, I gave in the previous editions the following 

 reasons for employing, in these cases, alcoholic rather than aqueous staining 

 media. Since, in most cases, treatment with alcohol forms part of the fixing 

 process, alcoholic solutions are logically indicated for staining. For by means 

 of them it is possible to avoid the bad effects that follow on passing delicate 

 tissues from alcohol into water, violent diffusive currents being thereby set 

 up which sometimes cany away whole groups of cells ; swellings being 

 caused in the elements of the tissues ; and, if the immersion in the aqueous 

 medium be prolonged, as is generally necessary in order to obtain a thorough 

 stain, maceration of the tissues supervening. But alcoholic staining fluids 

 have still other advantages ; they are in general more penetrating ; with them 

 alone is it possible to stain through chitinous integuments ; and, if it be de- 

 sired to stain slowly, tissues may be left in them for days without hurt. 



Applied to the case now under consideration, the preparation in toto of 

 objects protected by not easily permeable investments, this doctrine is evi- 

 dently a wise one. For such objects must necessarily be fixed by some highly 

 penetrating but not permanently hardening agent such as picric acid, and 

 must necessarily be washed out with alcohol ; and it is a good maxim for 

 tissues so fixed that an object that has once been in alcohol should not be 

 allowed to go back into water, if that can possibly be avoided. 



But in the case of structures that have been well fixed in a strongly and 

 permanently coagulating medium such as chromic acid, this precaution is 

 much less necessary. Sections of tissues that have been fixed for twenty- 

 four hours in Flemrning's solution may be passed with relative impunity 

 from absolute alcohol into an aqueous stain, and from that back again direct 

 into absolute alcohol. It is this property of tissues fixed in chromic solution 

 that determines me to recommend the practice of staining sections, instead 

 of staining objects in toto. As regards the quality of the stain, aqueous 

 solutions are in general the best. No alcoholic carmine or hematoxylin, for 

 instance, will give a stain equal in precision and delicacy to those given by 

 alum-carmine or Bohmer's hematoxylin. In a recent publication Mayer 

 himself states that he has somewhat cooled in his preference for alcoholic 

 staining media. 



For an excellent exposition of the principles underlying the practice above 

 recommended, the reader may consult with advantage the paper of Paul 

 Mayer, in Mitth. Zool. Stat. Neapel, ii (1881), p. 1, et seq. See also the 

 abstract in Journ. Roy. Mic. Soc. (N. S.), ii (1882), pp. 866881, and that 

 in Amer. Natural, xvi (1882), pp. 697 706, in which two last some im- 

 provements are mentioned which have been worked out since the publication 

 of Mayer's paper. 



