36 



THE AMERICAN MONTHLY 



[February, 



garis, without proper examination. 

 The two genera are, however, easily 

 distinguished by the position of the 

 eyes. Mr. Balen states that in Roti- 

 fer the eyes are on the probosis, very 

 far forward, while in Fhilodina they 

 are situated behind the calca, or cer- 

 vical. It is well to bear this dis- 

 tinction in mind, although it is one 

 distinctly set forth in all books treat- 

 ing of the Infusoria. 



Method of Imbedding. — The fol- 

 lowing method of imbedding in gum, 

 seems to have much to commend 

 it for delicate objects of which 

 sections are to be cut, although it re- 

 quires some time to prepare the speci- 

 mens for cutting. The great advantage 

 it seems to possess is that the tissues 

 become slowly but thoroughly pene- 

 trated with the mepium. 



Pure gum arable is dissolved in 

 water, forming a syrupy liquid. A 

 portion of this is poured into a watch- 

 glass, so as not to quite fill it, and six 

 or ten drops of glycerin are added. 

 The specimens to be cut are placed in 

 the fluid, and then the solution is al- 

 lowed to dry, which may require sev- 

 eral days. The gum becomes carti- 

 laginous, when it may be cut into 

 pieces enclosing the specimens. These 

 pieces are taken out and allowed to 

 dry still more. They will never be- 

 come hard, owing to the glycerin, 

 and may be kept indefinitely. The 

 drying may be hastened by artificial 

 heat, or by exposure to the sun. The 

 best sections seem to be obtained 

 when the pieces have had a week to 

 dry. The medium is transparent, so 

 that the objects may be examined 

 with the microscope before cutting, 

 and the sections made at exactly the 

 points desired. 



It will be remembered that a method 

 of imbedding proposed long ago was 

 to place the object in a solution of 

 gum until saturated with it, then par- 

 tially dry it until hard enough to be 

 plunged into alcohol. The alcohol 



removes the water from the gum and 

 hardens it. Whether the new method 

 above described is better in all cases 

 than this more expeditious one we do 

 not know, but in cases where it is not 

 desirable to use alcohol, or when the 

 object is very delicate, we have no 

 doubt it is preferable. 



Methods of Microscopical Work. 

 — The methods of preserving, stain- 

 ing and mounting objects in use at 

 the zoological station at Naples, have 

 already been published in several jour- 

 nals, but as some of them are of 

 special interest to our readers, we 

 briefly allude to them, taking the in- 

 formation from l\\& Journal Royal Mic. 

 Society. 



Picrosulphuric acid, a fluid intro- 

 duced by Kleinenberg, is used for 

 killing specimens. It kills quickly, 

 rapidly replaces the sea-water, and 

 may be directly replaced by alcohol, 

 and does not harden. It is useful for 

 embryos of vertebrates, fishes, and 

 Crustacea. It is composed of a sat- 

 urated solution of picric acid in water, 

 loo parts, by measure, concentrated 

 sulphuric acid, 2 parts. Filter, and 

 dilute with three times its bulk of 

 water. 



Alcohol is a valuable agent for kill- 

 ing, but it is not generally used in the 

 right way. The best way is to add 

 the alcohol to the water that contains 

 the specimen — an annelid, for ex- 

 ample — slowly. The animal then 

 remains extended and supple. 



Osmic acid is apt to blacken the 

 objects stained in it by continuing to 

 act after removal from the acid. Ob- 

 jects thus rendered worthless may be 

 restored by placing them in 70 per 

 cent, alcohol, with some crystals of 

 potassic chlorate, adding strong muri- 

 atic acid until chlorine is liberated 

 which bleaches the object. 



Corrosive sublimate in concentrated 

 solution is used for killing planarians, 

 corals, hydroids, echinoderms, minute 

 Crustacea, etc. 



