394 APPENDIX. 



surface, it is necessary to pour over it additional thick celloidin to insure 

 its complete enclosure. Expose to the air for a few minutes, until the cel- 

 loidin has set and the object is securely attached. 



6. Place block with object in 80 alcohol to harden celloidin, which will 

 require 12-24 hours, until sections are cut. Blocks are stored in 80 alcohol, 

 where they may remain for years. Should the embedded object separate 

 from the block, as sometimes happens during cutting, the surface of the 

 block must be cleaned, a new layer of thick celloidin applied, and the hard- 

 ening in 80 alcohol be repeated. In general, the smaller the object and the 

 harder the celloidin the thinner can sections be cut. The advantages of the 

 celloidin method include : ( i ) avoidance of heating the tissues, which if in- 

 judiciously done may distort or altogether ruin them ; (2) simplicity of pro- 

 cedure, neither embedding oven and accessories nor close attention being 

 required ; and (3) ready application of various methods of staining. The 

 chief disadvantages of the method are : ( i ) difficulty of obtaining extremely 

 thin sections; (2) impracticability of preserving the sequence of sections, 

 unless special means are employed ; (3) length of time ordinarily required 

 to prepare object for sectioning. All in all, the celloidin method is to be 

 preferred for routine histological work of the beginner, although for many 

 special purposes and for embryological material it is much less satisfactory 

 than paraffin. 



Paraffin Embedding. Although invaluable for work demanding 

 serial or very thin sections, the paraffin method requires some means of 

 maintaining a constant temperature. With care and attention this may be 

 done with a simple water bath, but for serious work a suitable embedding 

 oven is almost a necessity. This should be made of copper, have double 

 walls enclosing a water-space, and be supplied with a suitable door on one 

 of the larger sides of the rectangular box. The water-space must be pro- 

 vided with at least one opening large enough to admit the bulb of a gas- 

 regulator and the supporting perforated cork. A second aperture, with 

 tubular collar, should lead through the water-space from the top into the 

 interior of the warm chamber and serve for the passage and support of the 

 thermometer. The bulb of the latter should occupy the approximate centre 

 of the oven. One or two perforated copper shelves, not less than 6 cm. 

 apart, provide space for half a dozen or more embedding capsules. Since 

 the temperature to be maintained is from 50-55 C. , some form of "micro" 

 burner is desirable. If possible, a "blue flame" burner should be used to 

 avoid the gradual accumulation of soot deposited from the ordinary gas jet. 

 Even the miniature Bunsen burners often give too much heat unless the bot- 

 tom of the oven be raised sufficiently high. Adequate protection of the 

 flame from draughts is important. An .efficient regulator to control the flow 

 of gas is, of course, necessary. The usual inexpensive form (Reichert's) will 

 answer, a daily variation of one or two degrees ordinarily doing no harm. 

 Not infrequently the by-pass is too large and the opening must be reduced 

 before a satisfactory temperature is maintained. On installing a new oven, 

 usually some days are required in testing and adjusting it before valuable 

 tissue should be entrusted to prolonged embedding. 



Paraffin should be of two kinds, one the soft with a melting point of 

 45 C., and the other the hard melting at 54 C. It is important that 

 the paraffin be of good quality, that supplied by reliable firms being 

 chosen. The quantity used is so inconsiderable, that the slightly increased 

 cost is of little consequence in comparison with the satisfaction of having a 

 dependable article. Neither of the above grades is used alone, but a mixt- 



