578 EMBEYOLOGY OF THE LOWER VEETEBEATES app. 



carrier, especially if care has been taken to trim the surfaces of the block 

 parallel to the three chief planes of the embryo. 



Where greater accuracy is needed, as in the case of very small embryos, 

 they should be arranged in position in the melted paraffin with warm 

 needles under the prism binocular microscope. This may be done by 

 placing the watch-glass or other vessel on the top of a small flat copper 

 cistern full of water, provided with inlet and outflow tubes, and heated up 

 by contact with the top of the water-bath or hot stage. In the bottom of 

 the embedding vessel is placed a small plate of glass on the upper surface 

 of which are engraved parallel lines intersecting one another at right angles. 

 When the embryos have been accurately orientated with regard to the 

 engraved lines a stream of cold water is allowed to run through the cistern 

 and this causes the paraffin rapidly to solidify. When the block is quite 

 hard the glass plate is picked off and the ridges formed by its engraved lines 

 serve as accurate guides to the position of the embryo. 



Still greater accuracy is obtainable by arranging that the melted 

 paraffin in which the embryo is being orientated is already in its definitive 

 position on the holder of the microtome, the paraffin being kept melted as 

 long as necessary by an electric current passing through a loop of high 

 resistance wire. 1 



For the actual cutting care must be taken that the razor (solid ground) 

 or other knife has a very fine edge which does not show irregularities when 

 examined under the low power of the microscope. The blade should be 

 thoroughly cleaned with pure spirit before commencing work. If very 

 thin sections, e.g. of 1 /a in thickness, are required it is well to commence 

 with sections of 5 //., then without stopping to change to 4 /x, then to 

 3 [x, then to 2 jj., then to 1 /x — cutting a continuous ribbon throughout and 

 going ahead rapidly when the 1 ti sections are cutting properly. 



The celloidin method should be constantly used as a check on the 

 paraffin method. Where yolky eggs or embryos are being cut the 

 celloidin method gives the only trustworthy sections as by it the yolk 

 granules are held in position and prevented from sticking on the edge of 

 the knife, ploughing through the tissues and destroying much of the fine 

 detail, as is always liable to happen if paraffin alone is used under such 

 circumstances. 



In cases where there is no need for specially thin sections (say under 

 25 jj) a convenient method is that in which the celloidin block is hardened 

 by exposure to chloroform vapour and then cleared by immersion in 

 cedar-wood oil. 



The block of celloidin is usually fixed to a block of wood which is 

 gripped by the holder of the microtome. Care should be taken that such 

 wooden blocks are baked for several days so as to ensure their being 

 absolutely dry. Otherwise moisture will diffuse out and produce a milky 

 opacity in the celloidin which ought to be absolutely clear and transparent. 



Sometimes it will be found that the block becomes too hard and will 

 not cut properly, its edges frilling or breaking. This is sometimes due to 

 the presence of a trace of chloroform in the cedar oil used for clearing. 

 When this is the case the cut surface of the block should have perfectly 

 pure cedar oil applied to it with a brush just before each section is cut. 



1 A special apparatus for this purpose is made by the Cambridge Scientific 

 Instrument Company. 



