124 ROBERT CHAMBERS. 



12 mm. high, is placed on the microscope so that it may be moved 

 about with the mechanical stage. The chamber is roofed over 

 with a carefully cleaned coverslip, on the under surface of which, 

 the specimen is mounted in a shallow hanging drop of a phys- 

 iologically indifferent fluid. The dissecting needle is made by 

 drawing out one end of a piece of glass tubing and bending it at 

 right angles two or three millimeters from the pointed tip. The 

 needle-holder, a mechanism allowing of three movements, is 

 clamped to one side of the microscope stage and the needle is 

 adjusted so that it may project into the moist chamber with its 

 tip pointing up into the hanging drop. By proper adjustment the 

 cell to be dissected and the point of the needle are brought into 

 the same focal field. The three movements of the needle, per- 

 mitted by the needle-holder, and the two movements of the moist 

 chamber, by means of the mechanical stage, give the experi- 

 menter ample opportunity to carry on dissection under the 

 highest magnification of the microscope. 



THE INSTRUMENT. 



The holder accommodated to carry only one pipette or needle 

 is illustrated in Fig. I. It is useful in many ways and is com- 

 paratively inexpensive. Its disadvantage lies in the fact that 

 the resistance of the cell-substance to the needle may overcome 

 the inertia of the cell, so that, instead of tearing through the cell, 

 the needle simply drags it about. However, by careful manipula- 

 tion and by the possible use of a viscous dissection-medium much 

 can be done. Many marine ova and germ cells in general are 

 very soft in consistency and, if the drop in which they are im- 

 mersed be shallow enough, the surface tension of the drop flattens 

 them against the coverslip and is sufficient to hold the cell. The 

 single holder can also be successfully used for injecting material 

 into or extracting material out of a cell. In addition to the 

 movement of the needle, manipulation is greatly aided by the 



BULL., XXV., and (on protoplasm) in Amer. Jour. Physiol., XXXII., 1914, Cham, 

 bers (on the nucleus) in Science, XL.; Kite (structural transformations of blood 

 cells) in Jour. Inf. Dis., XV. 1915, Chambers (on the germ cell) in Science, XLL; 

 and (on protoplasm) in Lancet-Clinic, March 27, Cincinnati; Kite (on cell perme- 

 ability) in Amer. Jour. Physiol., XXXVII. 1917, Chambers (on protoplasm) in 

 Amer. Jour. Physiol., XLIII.; and (on the cell aster) in Jour. Exp. Zoo/., XXIII. 



