REACTIONS OF FIXATIVES WITH TISSUES AND CELLS 69 



It seems probable that the hpids of the cell-membranes act as 

 barriers. 



Fixatives penetrate more quickly into small than into large 

 pieces of tissue. This is of practical importance. It might be 

 thought a matter of indifference whether a large or a small piece 

 were cut out of an organism for fixation: attention could be con- 

 centrated on the edge of the large piece, which would be as well 

 fixed as the smaller. This is not so, for the following reason. 



Imagine two blocks of tissue, with opposite sides parallel. One of 

 them, block A, is exposed to the fixative on one side only (say the 

 lower side): the other block, B, is exposed on the two opposite 

 sides. 



Consider first the passage of the fixative into A. It may be re- 

 garded thus. The fixative instantly reaches the upper side at 

 infinitesimal concentration ; later at progressively higher concentra- 

 tions. For a considerable time the concentration at the upper side 

 will be below that at which protoplasm is fixed (that is, either 

 coagulated or changed into a stable gel). Meanwhile the substance 

 will be gradually penetrating at its fixative strength, according to 

 its K-value, and the line separating the fixed from the unfixed 

 tissue will be advancing at continually diminishing rate towards 

 the upper side, in accordance with the equation given on p. 37. 



In block B the course of events will be different. On each side 

 the substance would tend to penetrate at its fixative concentration 

 at the same rate as it did on one side of block A, but from the first 

 moment onwards there will be an infinitesimal contribution from 

 the opposite side, and this will continually increase. The concen- 

 tration at any given place at any given time will therefore be higher 

 than it would have been if only one side had been exposed to the 

 fixative. The thinner the piece, the more evident this effect will be. 

 It follows that a fixative will penetrate more quickly into a small 

 than into a large piece of tissue, and small pieces should therefore 

 be used unless there is some particular reason for not doing so. 



It is instructive to watch cells being fixed, and to record the 

 effects not only on the ground cytoplasm, but on every constituent 

 part. It was probably the transparency of living cells that held back 

 this kind of investigation in the early days of microtechnique. As 

 one optical method after another has been invented to overcome 

 this difficulty, so enthusiasts have hastened to apply the new 

 microscopes to problems of fixation. Not everyone who has 



