40 



FIXATION 



observations at 5^ hours. Apart from those for osmium tetroxide, 

 they would not have differed much if periods other than 5^ hours 

 had been chosen instead. 



It is to be remembered that acetic acid penetrated into a some- 

 what different gel from the others. 



It follows from the equation that a fixative with a K value of 

 I-o (chromium trioxide) will penetrate 20/x (the diameter of a large 

 cell) in I -44 sec. ; that is to say, it penetrates that distance at the 

 rate of 50 mm per hour, but the rate falls off so rapidly that in fact 

 it only penetrates i mm in an hour, and it takes 100 hours to pene- 

 trate I cm. As we shall see (p. 68), penetration into tissues is 

 slower than into gelatine/albumin gels, and it is obvious that the 

 internal parts of pieces of tissue several cm thick cannot be 

 effectively fixed even by the most rapidly penetrating fixatives. 



The results with gelatine/albumin gel agree in general with 

 Medawar's, so far as coagulant fixatives are concerned, but fixa- 

 tives penetrate more quickly into coagulated blood-plasma than 

 into the gel used in the experiments described here. 



We turn now from naked-eye observations to microscopical 

 study. 



The minute structure of protein coagula was first investigated 

 by the German botanist, Berthold,^^ in 1886. He believed that the 

 protoplasmic network seen in preparations of plant cells was a 

 coagulation artifact. He put a drop of egg-white on a slide and 

 noticed the formation of a microscopical network in it on the 

 addition of water (presumably by the coagulation of globulin and 

 ovo-mucoid). He also noticed the formation of separate granules on 

 the addition of an aqueous solution of iodine. 



Another German botanist, Schwarz,^^^ who was aware of 

 Berthold's findings, published the results of a much fuller investi- 

 gation in the following year. He added absolute ethanol, picric 



