88 IMBEDDING METHODS. 



wedge, five or six times as long as broad, the object being contained 

 in the broad part, and the edge turned towards the knife (see Fig. 4). 

 The sections are allowed to roll and come off as coils, the section of 

 the object lying in the outermost coil, which will be found to be a 

 very open one indeed, very nearly flat. Lay the coil on a slide 

 with this end downwards, warm gently, and the part containing the 

 object will unroll completely and lie quite flat. 



ANILE (Glandole duodenali, Napoli, 1903, p. 51) and VASTARINI- 

 CRESI (Mori. Zool ItaL, 1906, p. 164) lay a strip of wet filter-paper 

 on the block. 



A defect opposite to that of the rolling of sections is the compression 

 and the crumpling or puckering of sections, indicating that the 

 paraffin has been compressed by the knife instead of being merely 

 cut true by it. Such sections, 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. MAYER (Grundzilge, LEE and MAYER, 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 cut, the section is rolled off on to the surface of water. 



147. Cutting Brittle Objects (Collodionisation). 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 cutting 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. Roy. Mic. Soc., 

 1885, p. 738) gives the following directions : 



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



