377 
Dissection and Injection of Living Cells. 
of this being that one may readily change the pillar with its heavy 
base for the more easily transportable stage-clamping device. 
The necessity of having one or two instruments is, of course, 
conditioned by the type of work to be done. For picking up 
bacteria one is sufficient. For micro-dissection in experimental 
embryology a great deal can be done with one instrument, but for 
cell injection in general and for tissue-cell dissection two instru- 
ments are indispensable, so that two needles or a needle and a 
pipette may be manipulated simultaneously. 
When one instrument only is to be used the left-handed form 
(figs. 2 and 3e ) is probably the more convenient. For bacterio- 
logical work the right-handed form is preferred, as it can be 
swung around to fit on the left side of the microscope. When a 
pair is to be used the most convenient combination is a left- 
handed manipulator which clamps to the microscope stage, and 
a right-handed one attached to a pillar (fig. 3). 
3. The Setting-Up and the Working of the 
Instrument. 
Fig. 3 shows two instruments in place ready for work. They 
should be as close together as possible, so that the open end of the 
moist chamber need not be too wide to accommodate the needles. 
This leaves ample room on either side for the attachment of a 
mechanical stage. 
The instrument is provided with means for a preliminary 
adjustment of the needle in any direction. By these means the 
needle tip is brought into the field of a low-powered objective. 
Before centring the tip the bars which control the fine adjust- 
ments must be put into a state of tension by giving a few turns 
to the milled heads of each of the three screws. The needle tip 
is then more or less accurately centred, and finally raised close 
to the hanging drop. The instrument is now ready for action. 
The milled heads of the screws which control the lateral move- 
ments are provided with holes for rods to be used as levers. A 
most useful accessory is a wire-wound flexible shaft about 
2 ft. 6 in. long (fig. 3c), with a milled head at one end (fig. 3 a) and 
the other end attached to the screw controlling the up and down 
movement. Curving the shaft around one side of the microscope 
brings the control of this screw, which is the one most frequently 
used, close to that of the fine adjustment ol the microscope. The 
shaft also facilitates the use of both hands for the various move- 
ments of the one instrument. 
Another useful accessory is a brass collar 1J in. long (fig. 3 1 ) 
with a spring which projects into its lumen through a slot. The 
shank of the needle is slipped through the collar and the screw, 
clamping the spring, tightened sufficiently to enable one to slide 
