Casting block 



PARAFFIN SECTIONS 



103 



tive as a bunsen burner; (3) a slab of plate 

 glass; (4) a wide-moutli, oye-di-oppor ty])e 

 pipet. It is presumed that tlie object itself 

 is in the oven, which also contains a sup- 

 ply of molten medium. It must l)e empha- 

 sized that an object cannot successfully 

 be impregnated with one kind of wax and 

 embedded in another. Next wet the under- 

 side of the bottom of the paper l)()x and 

 press it into contact with the plate-glass 

 slab. Then take from the oven (Fig. 52) 

 a beaker of molten embedding material 

 and fill the little paper box to the brim. 

 The eye dropper is then heated in the 

 flame to a temperature well above that at 

 which the wax will melt, and is used to 

 pick up the object from its own dish (Fig. 

 53) and to transfer it to the paper box. 

 By the time this has been done, a layer of 

 hardened wax will have been formed at the 

 bottom of the paper box, so that the ob- 

 ject will rest on the layer of solidified wax 

 with a molten layer above. It will almost 

 invariably happen that the surface has 

 also cooled, so that a crust of cool wax will 

 have been carried down with the object 

 in the box. It is essential to get rid of this 

 if the wax is to adhere through section cut- 

 ting, and the pipet is again heated, used to 

 melt the entire surface of the wax (Fig. 

 54), and to maneuver the object into the 

 approximate position in which it is re- 

 quired to lie in the finished block. Then 

 blow on the surface until the wax is suffi- 

 . ciently solidified to enable you to pick up 

 the box carefully and (as shown in Fig. 55) 

 to hold it on the surface of the water used 

 for cooling. With most wax media it is 

 desirable to cool the block as rapidly as 

 possible; it should never be permitted 

 to cool in air. It cannot, however, be 

 pushed under the surface of the water, or 

 the molten center is liable to break 

 through the surface crust and thus destroy 

 the block. After it has been held in the 

 position indicated until it is fairly firm 

 throughout, it may be pushed under the 

 surface to complete the cooUng. 



The block may be left in water for an>- 

 reasonable length of time; but if it is to be 

 stored for days or weeks it is better kept 

 in a 5% solution of glycerol in 70' t alco- 

 hol. There seems to be a widespread de- 

 lusion that because an object must be 



perfectly dehydrated before being impreg- 

 nated with wax, it must subse(iuently be 

 kept out of contact with fiuids. Nothing 

 could l)e further from the truth. As will 

 be discussed later, when dealing with the 

 actual techiiiciue of sectioning, it is often 

 desiral)le to expose a portion of the object 

 to be sectioned and leave it under the sur- 

 face of water for some days, in order to get 

 rid of the brittleness which has been im- 

 parted through the embedding process. 

 Blocks which have been stored dry for a 

 long jjeriod of time should always be 

 soaked in a glycerol-alcohol mixture for at 

 least a day before sectioning. 



It is, in any case, undesirable to section 

 a block as soon as it has been made, for 

 it is necessary for successful sectioning 

 that the block should be the same temper- 

 ature throughout. If a block is made in the 

 evening, it is better to take it out of the 

 water and to leave it lying on the bench 

 overnight in order that the temperature 

 may be stabilized. Assuming, however, 

 that we have such a block at hand, the 

 next thing to do is to mount it in what- 

 ever holder is to be used. 



Choice of a Microtome 



Microtomes may be broadly divided 

 into two classes. In the first of these the 

 block remains stationary while the knife 

 is moved past it; in the second group are 

 those in which the block moves past a 

 stationary knife. The first class (an exam- 

 ple is shown in Fig. 56) is made by several 

 manufacturers but is rarely used for the 

 preparation of serial sections. They have 

 the advantage that relatively large blocks 

 may be cut, but thej' have the disadvan- 

 tage that no ribbon can be ol)tained which 

 is broader than the width of the knife. 

 This microtome will not be discussed fur- 

 ther in the i)resent place, for a detailed 

 description of its use is given in the next 

 chapter on nitrocellulose sections, with 

 which this type of microtome is often to be 

 preferred. 



A Minot, or rotary microtome, is shown 

 in Fig. 57. In this type of microtome the 

 rotation of the large wheel causes the 

 block holder to move vertically up and 

 down, in most instances tlirough a dis- 

 tance of about three inches. The portion 



