THE MICROVIVISECTION METHOD. 133 



wire. To produce chemical effects in a cell, McClendon ('09) 

 used a copper wire ground to a point and further sharpened by 

 erosion in acid. The chemical injury produced by metal needles 

 limit their use for the dissection of living cells. 



For experiments in cutting cells, one must take into account 

 the fact that many cells will lose their more or less fluid contents 

 if a gap be torn in their surface. If, however, the side of a fine 

 needle can be brought to press the cell against the coverslip, a 

 constriction will result which may cut the cell into two intact 

 pieces. For cutting soft-bodied ova and protozoa this can be 

 done with a form of needle depicted in Fig. 7. The needle tip 



FIG. 7. Needle with tip curved back to almost an horizontal plane, a, lateral 

 view, b, view from above. 



is bent back rather than forward to lessen the chances of breaking 

 the tip when adjusting the needle in the moist chamber and also 

 when the needle needs to be cleaned (see below). The tip should 

 not point directly back but slant off to one side (see Fig. 7, b), so 

 that the shank of the capillary will not be in the way of the light 

 coming up from below. The angle at which the needle is bent 

 must be ascertained by successive trials, for, if it be bent too near 

 to a horizontal plane, the tapering tip will bend away from the 

 cell instead of cutting into it. If the angle be too far from the 

 horizontal plane, the cell will slip along the needle and escape. 

 When the proper angle is obtained, one can cut Arbacia and 

 Asterias ova into pieces of almost any required size with con- 

 siderable ease. A clean cut through an unfertilized sea-urchin 

 egg or a fertilized egg, before the fertilization membrane has 

 become too tough, can be best made by bringing the egg between 

 the horizontal arm of the needle and the lower surface of the 

 hanging drop (Fig. 8). On lowering the needle out of the drop, 



