174 Wilson: A new System of obtaining directing-marks. XVII, 2. 



Tlie soiirce of heat is now removed from under tlie glass, and 

 as soon as the Strands have again solidiüed and tlius become adherent 

 to the glass, one of tbe embedding-bars is placed with oue arm 

 exactly along one of the side-lines of the quadrilateral outline of 

 the base-plate (or indeed any one of the parallel lines will do), and 

 witli its other arm crossing at right augles to the series of parallel 

 lines, and slightly overlapping the projecting ends of the uerve- 

 strands lying along these. The subsequent embedding will be so 

 carried out that this cross arm of the embedding-bar will limit the 

 basal plane of the future paraffin block, corresponding to, or parallel 

 with, the plane of sectioning. The other L-shaped embedding-bar 

 may now be placed in position, without disturbing the first-laid one, 

 and also slightly overlapping the other ends of the nerve-strands. 

 A moderate weight, say of about a kilo, and best in the shape of 

 a metal plate, is to be laid upon the npper snrfaces of the em- 

 bedding-bars, and the under surfaee of the base-plate is once more 

 heated up to the melting-point of the parafhu , and either again 

 allowed to cool, or the process of embedding carried out forthwith. 

 This weighting of the embedding-bars, while the base-plate is being 

 heated up, ist to allow of a complete flattening out of the ends of 

 the Strands of tissue which are overlapped by the bases of the em- 

 bedding-bars. The nerve filaments, if suitable ones have been chosen 

 for the process, are so delicate that, when thus gently tlattened out 

 under pressure, their thickness is inappreciable and does not atfect 

 the perpendicularity of the surfaces of the embedding-bars. ^ 



^) If, however, this quite inappreciable amount of flattened-out material 

 beneath the embedding-bars, be yet objected to as wrong in principle and 

 dangerous in practice, another method of ensuring the stability in position 

 of the nerve tilaments during the subsequent process of embedding, may 

 be resorted to. After obtaining perfect coincidence of the nerve Strands 

 with the orientation lines on the glass, and before placing the embedding- 

 bars in Position, both ends of the nerve-strands (cut so as not to project 

 beyond the limits oi the embedding-area, and molted on the glass) are 

 covered by two small and moderately thin pieces of lead, such as printers' 

 space-types, and these are pressed down temporarily by means of a small 

 weight resting upon them while the base-plate is being heated up. The 

 small pieces of lead are now left undisturbed in position close to the 

 limits of the embedding-area; the embedding-bars are placed in position 

 around the area; and the embedding of the object is carried out with the 

 former still in situ. The small lead blocks can easily be picked out from 

 the surfaee of the block of paraffin before trimming it for the microtome. 

 This expedient somewhat reduces the available Space on the floor of the 



