86 FIXATION 



When living cells are put in hypotonic solutions, those that are 

 dividing have a tendency to swell strongly, and mitosis often comes 

 to a halt at prometaphase or metaphase; the chromosomes are 

 widely dispersed. ^^^ Cells may be fixed in this condition, which is 

 especially favourable for the counting of chromosomes.^^^'^^^'^^^' 

 An aqueous solution of sodium chloride at 0-3 to 0-5% is suitable 

 for mammalian cells. ^^'' 



There is no evidence that it is ever advantageous to dissolve 

 fixative substances in complex physiological saline solutions. 



Urea ^ and saponine ^^^ are occasionally added to fixative solu- 

 tions. Their effects, if any, have not been critically analysed. There 

 is no concrete evidence that the reduction of surface tension is 

 helpful. 



It is desirable that the whole subject of non-fixative substances 

 in fixative solutions should be reinvestigated. Some of the most 

 important problems remain unanswered. In the absence of in- 

 different salts, do cells in the interior of pieces of tissue actually 

 swell, burst, and then shrink? Do the indifferent salts exert their 

 effects outside or inside the cell, or first outside and then inside? 

 We simply do not know. 



In the early days of microtechnique it was important that tissues 

 should be hardened so that they could be sectioned easily by hand. 

 With the introduction of effective embedding media, hardening 

 became less important. Nevertheless, the physical properties of 

 fixed tissues remained and still remain important. It is necessary 

 that the fixative should give them such a consistency that when 

 they have been embedded, they can be cut into sections easily. 

 They must not be friable, brittle, or very hard. Much depends on 

 the process of embedding. Some tissues, such as the lens of the 

 eye or thick pieces of voluntary muscle, are difficult to section in 

 parafiin but easy in collodion. In such cases the process of em- 

 bedding makes more difference than fixation. 



It would be useful to have quantitative information about the 

 physical properties of various tissues at all stages of microtechnique 

 up to embedding. Unfortunately the available data are meagre. 

 Wetzel ^^^ studied the elasticity of the belly-muscle of the cat. He 

 cut out a long piece and put it in a fixative solution. He then 

 attached it by one end in such a way that it was held horizontally. 

 A weight attached to the free end caused it to sag. The coefficient 

 or modulus of elasticity was calculated from a formula involving 



