ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 871 



added with a pipette, and as soon as chlorine (easily recognized by- 

 its greenish-yellow colour) begins to be liberated, the whole gently 

 shaken. As soon as the bleaching is finished the objects are removed 

 to pure alcohol. By this method Dr. Mayer has been able in half a 

 day to restore large Pelagia, Carinaria, Rhizostoma, &c. Small objects 

 generally require a shorter time and less acid. The process can be 

 greatly accelerated by heating on a water-bath. 



Using Sapphirina as a test-object, Dr. Mayer found that the lustre 

 which characterizes the living animal entirely disappeared by the 

 bleaching process. As this lustre, which has its seat in the epidermis, 

 depends on the interference of light, it is evident that the cells had 

 undergone some change, but a change so slight that the tissues could 

 hardly be said to have been injured for histological purposes ; besides, 

 the removal of the osmic acid leaves the animal in a good condition 

 for staining. 



Dr. Mayer's experience with Sapphirina appears to support him 

 in the following conclusions in regard to the nature of the action of 

 osmic acid, viz. that the hardening effect of the acid is due to the 

 formation of inorganic precipitates within the tissues. This is made 

 evident by the fact that the animal becomes soft and flexible as soon 

 as these precipitates are removed by bleaching. 



This method of bleaching has been used by Dr. Mayer for 

 removing natural pigment. Alcoholic preparations of the eye of 

 Mysis, for instance, can be fully bleached in toto, but with better 

 success by operating with single sections. To avoid swelling, which 

 is apt to arise by the use of aqueous fluids, staining media of an 

 alcoholic nature should be used. 



7. Chromic Acid. — Chromic solutions have, in common with osmic 

 acid, the peculiarity of hardening by virtue of the chemical combina- 

 tions which they form with cell-substances, and all the consequent 

 disadvantages with respect to staining. The use of chromic acid in 

 the Zoological Station of Naples may be said to have been largely 

 superseded by picro-sulphuric acid, corrosive sublimate, and Merkel's 

 fluid, for it is now seldom used except in combination with other 

 fluids.* It is sometimes mixed with Kleinenberg's fluid, for example, 

 when a higher degree of hardening is required than can be obtained 

 by the use of the latter fluid alone. It is a common error to use too 

 strong solutions of chromic acid, and to allow them to act too long. 

 Good results are in some cases obtained when the objects are treated 

 with a weak solution (-5— £ per cent.) and removed soon after they are 

 completely dead. 



8. Merkel's Fluid. — 



Platinum chloride dissolved in water 1:400 



Chromic acid „ „ 1:400 



* Dr. W. Pfitzner (Morphol. Jabrb., vii. (1882) p. 731) has recently made use 

 of chromic acid followed by (1) osmic acid, or by (2) chloride of gold, formic acid 

 and safranin (or hematoxylin) for tbe demonstration of nerve- terminations. 



Flemming believes that chromic acid is one of the most reliable fixing re- 

 agents for the karyokinetic figures, and has proved that objects hardened iu this 

 acid can be beautifully and durably stained, ante, p. 715. 



