104 MICROSCOPICAL RESEARCH IN THE 



the blackening process can be arrested by certain staining media, 

 but it is certain that picro-carmine will not always do this, as some 

 of my preparations of Dicyemidce, show. It is therefore a very 

 important step which Dr. Mayer has taken in finding a method of 

 restoring such objects. The method* is as follows: — The objects 

 are placed in yo per cent, or go per cent, alcohol^ and crystals of 

 potassic chlorate (KCIO^) shaken mto the liquid until the bottom of 

 the vessel is covered; then a few drops of concentrated hydrochloric 

 acid\ are added ivith a pipette, and as soo?i as chloi'ine (easily recog- 

 nised 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 Felagia, Carinaria, Rhizostoma^ 

 etc. Smaller objects generally require a shorter time and less acid. 

 The process can be greatly accelerated by heating on a water-bath. 



Using Sapphii'ina as a test-object, Dr. Mayer found that the 

 lustre which characterises 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 histologi- 

 cal 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. 



* A slightly modified form of the method originally given in MUll. Arch., 1874, 

 p. 321. 



t Nitric acid may be used instead of HCl. 



