110 CHAPTER VIII. 



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. 



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

 compression and the crumpling or puckering of sections, 

 indicating that the paraffin Eas 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 

 l>y the paraffin being too soft. To prevent it, correct the 

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

 If the crumpling has not gone so far as to cause the folds of 

 the sections to adhere to one another, the sections may be 

 perfectly cured by flattening on water ; see below, ' ' Section- 

 flattening/' 138. 



Devices for heating or for cooling the knife, with a view to the improve- 

 ment of cutting, have been described ; see VAN WALSEM in Zeit.f. wiss. Milt., 

 xi, 2, 1894, p. 218 ; also Jung's price-list. I have myself sometimes found 

 it advantageous to warm the knife. 



136. Collodionisation of Sections. Some objects are by 

 nature so brittle that, notwithstanding all precautions taken 

 in imbedding and previous preparation, they break or crumble 

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

 impossible to mount them in the ordinary way without some 

 impairment of their integrity. Ova are frequently in this 

 case. The remedy for this state of things consists in cover- 

 ing 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 of even the most fragile sections in 

 a wonderfully efficacious way ; and the same treatment ap- 

 plied to tissues which are not specially fragile will enable the 

 operator to cut sections considerably thinner than can be 

 obtained in the usual way. BUTSCHLI has obtained in this 

 manner sections of less than 1 /m in thickness. 



