INTRODUCTORY. 7 



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 carry away whole groups of cells ; swel- 

 lings being caused in the elements of the tissues ; and, if the 

 immersion in the aqueous medium be prolonged, as is gene- 

 rally necessary in order to obtain a thorough stain, macera- 

 tion of the tissues supervening. But alcoholic staining fluids 

 have still other advantages - j they are vastly more penetrating ; 

 with them alone is it possible to stain through chitinous 

 integuments ; and, if it be desired to stain slowly, tissues may 

 be left in them for days without hurt. 



Applied to the case now under consideration, the prepara- 

 tion in toto of objects protected by not easily permeable 

 investments, this doctrine is evidently 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 Flem- 

 ming'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 recom- 

 mend the practice of staining sections, instead of staining 

 objects in toto. 



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. Hie. Soc. (N. S.), ii (1882), pp. 866881, and 

 that in Amer. Natural., xvi (1882), pp. 697 706, in which 

 two last some improvements are mentioned which have been 

 \\orked out since the publication of Mayer's paper. 



