14 



Sections from Double-embedded Material 



General Principles 



The onl}' purpose of embedding objects 

 first in celloidin and then in paraffin is to 

 secure serial sections of material which 

 cannot be handled by the paraffin method 

 alone. This limits its utility to small ob- 

 jects which cannot with ease be oriented 

 in paraffin, or alternatively, to small ob- 

 jects of which it is quite essential to obtain 

 series, and which like many small arthro- 

 pods, cannot be retained sufficiently firmly 

 in a wax matrix to permit of sections being 

 obtained. 



It is possible to impregnate an object 

 with celloidin (as described in the last 

 chapter) and then to embed the impreg- 

 nated object in paraffin. There are, how- 

 ever, much better ways available which 

 shorten to a considerable extent this long 

 process. These methods are based on 

 the original suggestion of Peterfi 1921 

 (23632, 38:342) that a solution of celloi- 

 din in methyl benzoate could profitalily 

 be substituted for the more conventional 

 solutions. 



By this method the small objects are 

 dehydrated in the manner described in the 

 last chapter, just as much care being 

 necessary as though one were running a 

 straight celloidin impregnation, but are 

 then passed from the absolute alcohol- 

 ether mixture to a celloidin solution con- 

 taining methyl benzoate or methyl saU- 

 cylate. These solutions mostly contain 1 % 

 of celloidin. Formulas for some of them 



will be found in Chapter 17 under the 

 heading E 22.1. The advantage of methyl 

 ester solutions is that the celloidin does 

 not contract so much on hardening and 

 the solutions may be dropped directly into 

 a solution of chloroform to produce a solid 

 block. Minute objects — usually small 

 arthropods, invertebrate larvae, or proto- 

 zoans — are first impregnated with the 

 methyl ester solution, and then dropped 

 into a beaker of anhydrous chloroform. 

 Solidification is almost instantaneous and 

 the little globule containing the object 

 may be removed after five or ten minutes. 

 As there is usually difficulty in orienting 

 these minute objects, the author prefers 

 to color the celloidin-embedding medium 

 with the addition of 0.1% of eosin. This 

 enables the block to be trimmed to a rec- 

 tangular shape which may itself be orien- 

 tated without difficulty, for it is now 

 clearlj^ visible in paraffin. 



Chips of the embedding medium are 

 added to the chloroform, and this mixture 

 is transferred to the oven until it is fluid. 

 The small block is then removed, placed 

 in i)ure embedding medium for as long as 

 is necessary, and then made into a paraffin 

 block as described in Chapter 12. These 

 generahties are sufficient to introduce the 

 process which is much better described in 

 the form of the typical preparation which 

 follows. 



Typical Example 



Prep.\r.\tion of a Series of Sections, Intended for Reconstruction, of a 



Pluteus Larva of Echinus 



Echinoderm larvae are among the most sections, and tlic only method by wliicli 

 difficult objects from which to cut perfect their anatomy may reasonably be studied 



153 



