134 ROBERT CHAMBERS. 



the arm of the needle passes through the egg cutting it cleanly 

 in two. 



A good needle, when once made and proven to be resilient and 

 strong, may be used repeatedly. Mucinous material which soon 



FIG. 8. Side view of moist chamber to show method of cutting an egg cell in 

 two with needle shown in Fig. 7. 



covers its tip so as to render it useless can be got rid of by im- 

 mersing the needle successively in a strong acid and a strong 



alkali. 



CELL INJECTION. 



Barber, in his two papers ('n and '14) gives a full description 

 of his admirable mercury method for injection. With it one can 

 inject microscopically minute doses of a solution into any part 

 of a cell. The advantage of the pressure produced by the mer- 

 cury over that by air lies in the slow driving force of expanding 

 mercury which can be almost instantly arrested when the dose 

 injected is considered sufficient. On the other hand, if air pres- 

 sure be used, the air in the injection tube will compress enor- 

 mously as the resistance of the bore at the tip of the pipette is 

 too great to overcome. If the injection does occur it may go 

 with a rush usually followed by a volume of air which destroys 

 the cell being experimented upon. Notwithstanding this, it is 

 surprising what delicate work may be done simply with air pres- 

 sure. An example of this is the extraction of the nucleus from a 

 mature A rbacia egg and its subsequent injection into another egg. 

 For the injection of substances, however, where the amount must 

 be strictly controlled, the mercury method is essential. 



The extraction of material out of a cell is comparatively easy 

 to carry out, as the capillary attraction in the lumen of a micro- 

 pipette (the inside diameter of which may be anywhere from 2 

 micra up) is quite sufficient for the purpose. For injection pur- 

 poses the difficulty lies in securing the force to overcome this 

 capillary attraction. 



