THE PARAFFIN METHOD 



113 



agent is used for both killing and fixing. Zoologists, from humane mo- 

 tives, may use chloroform for killing, while other reagents are used for 

 fixing. In fixing root-tips, anthers, and other material for a study of 

 mitotic figures, it is necessary that killing be very prompt. In a weak 

 solution of chromo-acetic acid, nuclei which have begun to divide may 

 complete the division, although the reagent might hinder nuclei from 



Fig. 20. — Spencer rotary microtome with electric motor, and Land's apparatus for temperature 

 control. 



entering upon division. A strong chromo-acetic solution will increase 

 the number of mitotic figures, and the chromo-acetic-osmic solution, 

 given on page 28, will still further increase the number, and the pro- 

 portion of anaphases w^ill be greater. 



Take the kilhng and fixmg fluids into the field. If one waits until 

 the material is brought to the laboratory there may be some fixing, 

 but it will, in many cases, be too late to do much killing. Material 

 which has begun to wilt is not worth fixing. Material like Spirogijra, 

 however, may be wrapped in several thicknesses of newspaper, placed 



