REACTIONS OF FIXATIVES WITH TISSUES AND CELLS 67 



Hardy worked with the intestinal epithehum of the woodlouse 

 Oniscus (fig. 6) and with various cells of the frog and mammals. 

 'On the whole', he remarked, 'living cell-substance does react to 

 fixatives just as does solid or 

 fluid soluble colloid.' By solid 

 or fluid colloid he meant pro- 

 tein gels or sols. 



Since tissues become opaque 

 when invaded by coagulant 

 fixatives, it is possible to ob- 

 tain information about their 

 rate of penetration. If mam- 

 malian liver or some other 

 rather homogeneous organ be 

 chosen, the line of demarca- 

 tion between fixed and unfixed 

 tissue is often extraordinarily 

 clear and regular (fig. 5, a), 

 though the fixative runs more 

 quickly along the loose tissue 

 accompanying large blood- 

 vessels than through compact 

 masses of cells. It makes little 

 difference whether excised 

 blocks of liver-tissue be used, 

 or whole lobes; the latter, 

 however, give the clearest 

 results. It is evident that the connective tissue sheath of the organ 

 forms a negligible barrier. 



The earlier work on this subject ^^^' ^^^' ^^"^ was done before 

 Medawar ^^"^ had disclosed the principles governing the diffusion of 

 fixatives into protein gels (p. 37). Experiments on the penetration 

 of coagulant fixatives into whole liver-lobes of the rabbit prove 

 that the same principles govern penetration into tissues. ^^ It is 

 only necessary to suspend liver-lobes by threads in a large volume 

 of the fixative under test, and to remove these at measured inter- 

 vals and cut them across. The thickness of the coagulated tissue 

 may be measured rather accurately. The figure obtained is not 

 necessarily the same as the distance penetrated into the fresh 

 tissue, because the fixative may change the volume of what it 



FIG. 6. A cell from the intestine of Onis- 

 cus (woodlouse), fixed by mercuric 

 chloride (saturated solution in o-6°o 

 sodium chloride); I'4jli paraffin section, 

 dyed with iron haematein: to show the 

 coagulation of protoplasm. 

 (From Hardy.213) 



i 



