SECTION CUTTING AND PERMANENT MOUNTS. 107 



2. Longitudinal tangential section, which is parallel to a 

 plane tangent to the cylinder, and cuts the latter near the sur- 

 face. 



The razor must always be keen-edged and should be stropped 

 frequently to keep it thus, and, when necessary, honed. It is 

 impossible to cut a thin section with a dull razor. A razor flat 

 on one side is the best. It should always be cleaned after cut- 

 ting sections. It should be pushed, rather than drawn, through 

 the object, with an oblique or sliding motion, even and steady 

 and never with a to-and-fro or sawing motion. 



Sections may be cut free-hand or by the use of a so-called 

 microtome or section-cutter, a machine in which the object is 

 clamped in a jaw, which is raised by an accurate screw, while 

 the razor, either held by the hand or clamped in a carriage, 

 slides through it, giving very even and thin sections. 



FREE-HAND SECTIONS. If the object is large it is held in the 

 left hand between the thumb and forefinger, the latter being 

 extended slightly, so as to form a rest for the razor-blade, which 

 is held in the right hand. Small objects are held in elder or 

 sunflower pith, which is split longitudinally in halves. The ob- 

 ject is left protruding slightly, or both object and pith are cut 

 together. 



In most cases it is best, in cutting, to keep the knife-blade wet 

 with alcohol or a mixture of equal parts of alcohol and glycerin. 

 Sections of fresh tissues or of those that have been kept in any 

 of the preserving fluids should, immediately after cutting, be 

 transferred best by means of a camel's hair brush to water 

 or alcohol, otherwise air will get into the cells and seriously 

 impair the value of the section for study. In regard to cutting 

 sections with the microtome, practice under the eye of an in- 

 structor is the best teacher. 



While in the majority of instances, certainly in those men- 

 tioned in this book, the method of cutting sections above 

 described gives satisfactory results, in the case of very delicate 

 objects, such as anthers, very young ovaries, longitudinal sec- 

 tions of root tips, etc., more careful manipulation is required. 

 The objects are imbedded either in paraffin or celloidin.* The 

 paraffin method requires less time and is preferred by many to 

 the celloidin method, especially in animal histology. Celloidin 

 is especially applicable to delicate plant tissues. The two 

 methods are described in Sedgwick and Wilson's General 

 Biology, as follows : 



PARAFFIN METHOD. After hardening and staining, the object 

 is soaked in alcohol (95 per cent, or more) until the water is 

 thoroughly extracted (2 to 12 hours, changing the alcohol at 

 least once), then in chloroform until the alcohol is extracted 



"Celloidin is the best quality of gun-cotton, and occurs in the market in pieces 

 looking somewhat like cartilage. 



