EMBEDDING IN CELLOIDIN AND PARAFFIN 723 



watch glasses preferably solid ones is placed on this, each contain- 

 ing paraffin wax, and a point is found on the slab at which the wax is 

 melted and no more, the next watch glass containing solid wax. The 

 specimens are dropped into the wax with a pipette, and are readily 

 accessible. 



The double embedding process should always be used when dealing 

 with a part which contains chitin, such as the proboscis, or when 



cutting sections of the thorax to show the presence of 



_ . , ...... Double embedding 



filanae. It is the only method by which serial sections 



of whole flies can be cut. The principle of the method is to impregnate 

 the parts with a solution of celloidin, and then to embed in paraffin. 

 The celloidin supports the tissues and holds them together, while the 

 paraffin enables serial sections to be cut in the ordinary way. The prepa- 

 ration is brought into absolute alcohol after fixation, and then placed for 

 eight to twelve hours in a mixture of equal parts of alcohol and ether. 

 It is then transferred to a one per cent solution of celloidin in alcohol 

 and ether for ^twenty-four hours, and to a three per cent solution for the 

 same period. It is an advantage to replace the thin celloidin with the 

 thick very gradually, adding a little now and then, till finally the whole is 

 changed into a fresh three per cent solution. In cases where serious 

 difficulty in cutting sections is met with, it may be necessary to pass 

 from one-half per cent celloidin to four per cent over a period of four 

 days. When impregnated with the celloidin the tissue has to be 

 placed in cedar oil, and in order to avoid exposure to the air while 

 this is being done it is best to pour the oil into the tube with the 

 celloidin, decanting it carefully down the side. In a day or less, the 

 tissue, which at first lies at the junction of the two fluids, will sink 

 into the oil. It should be left for some hours after this, to ensure 

 thorough clearing. It is then embedded in paraffin in the ordinary 

 way, and for as short a time as possible, as both the chitin and the 

 celloidin, get very hard if heated much. 



It is often extremely difficult to get a good series of sections of chiti- 

 nous parts, and many trials have to be made before the correct method 

 is hit upon. If the preparation is kept too long in celloidin the soft 

 tissues shrink away from the chitinous framework, which also becomes 

 brittle, while if it is not kept long enough the parts will not remain coher- 

 ent when the razor passes through them. The amount of time for the 

 paraffin bath is the most difficult factor to determine. If the material is 

 left in it too long the chitin becomes brittle and the finer details of the 

 soft parts are lost through shrinkage, while if the time is too short the 



