IMBEDDING METHODS. 95 



besides showing creases or folds, have a smaller area than 

 that of the block from which they are cut. This is a bad 

 fault, for the compression may obliterate important cavities 

 or efface important limits between cell-layers, etc. It may 

 be caused by a badly cutting knife, and is very easily caused 

 by the paraffin being too soft. To prevent it, correct the 

 knife or cool the paraffin, or re-imbed in harder paraffin. 



Very large sections tend to form folds on the knife, and are difficult to 

 remove from it. Maybe (Grundziige, Lee and Maybe, p. 94) gets 

 them to wrap themselves round a glass or gelatin tube laid on the block 

 just in front of the knife-edge and rolled forwards as it progresses. 

 When out, the section is rolled off on to the surface of water. 



147. Cutting Brittle Objects (CoUodionisation). — Some objects 

 are by nature so brittle that they break or crumble before 

 the knife, or furnish sections so friable that it is impossible 

 to mount them in the ordinary way. Ova are frequently in 

 this case. A remedy for this state of things consists in 

 covering the exposed surface of the object just before cut- 

 ting each section with a thin layer of collodion, which serves 

 to hold together the loose parts; and will enable the 

 operator to cut sections considerably thinner than can be 

 obtained in the usual way. 



The primitive form of the process was to place a drop of 

 collodion on the free surface of each section just before 

 cutting it. But this practice has two defects ; the quantity 

 of collodion employed sensibly softens the paraffin, and the 

 thick layer of collodion when dry causes the sections to roll. 



Mark {Amer. Natural., 1885, p. 628; cf. Journ. Eoy. iJic. 

 Soc, 1885, p. 738) gives the following directions : 



" Have ready a little very fluid collodion in a small bottle, 

 through the cork of which passes a small camel-hair brush, 

 which just dips into the collodion with its tip. The collodion 

 should be of such a consistency that when applied in a thin 

 layer to a sui'face of paraffin it dries in two or three seconds 

 without leaving a shiny surface. It must be diluted with 

 ether as soon as it begins to show signs of doing so. 



"Take the brush out of the collodion, wipe it against the 

 neck of the bottle, so as to have it merely moist with collodion, 

 and quickly pass it over the free surface of the preparation. 

 Care must be taken not to let the collodion touch the vertical 



