466 DEVELOPMENT OF THE CHICK. 



under the salt solution, and with the other hand work a needle 

 very cautiously along the edge of the blastoderm, between it 

 and the vitelline membrane, until the blastoderm is quite detached. 

 The adhesion only occurs near the 'periphery. This operation 

 may be done in a watch-glass, but if the specimen is to be mounted 

 whole, it should now be floated on to a slide before fixation, in 

 order to become well .flattened. The fixative should be added 

 from a pipette until the whole is well covered. 



The length of time of fixation varies with the reagent em- 

 ployed. Osmic mixtures act very quickly ; sublimate in 

 half an hour, Perenyi's fluid in an hour, picro-sulphuric in 

 six hours. Wash well after fixing and transfer to increasing 

 strengths of alcohol. Keep in 75 per cent. 



For staining, weak borax-carmine is most generally useful, 

 and the embryos can be left in it over night. Grenacher's 

 Hcematoxylin serves equally well. Differentiate with acid 

 alcohol, dehydrate, clear and mount in balsam. 



To prepare Sections : 



Fix, harden, stain, and dehydrate as before. ' Cut away the 

 blastoderm from around the embryo. Place in pure cedar- 

 wood oil for an hour, then transfer to a mixture of cedar-oil 

 and soft paraffin at the temperature of the bath for half an hour, 

 and again to pure paraffin for one to four hours, according 

 to size. The paraffin should have the lowest melting-point 

 consistent with setting firmly at the temperature of the room. 

 A paraffin melting at 54 C. will be found generally useful. 



Prepare a small paper box by folding up the ends of greased 

 paper, or use a couple of L-shaped pieces of metal placed together, 

 so as to form a shallow trough. Warm a needle and pair of 

 forceps. Pour the paraffin into the box or trough, insert the 

 embryo, orientate quickly with needle and forceps, and lower the 

 trough to half its depth into a dish of cold water. Allow to set 

 thoroughly. Remove the block so formed. Trim it with a 

 sharp knife so as nearly to expose the embryo, and proceed to fix 

 it on the carrier of the microtome and cut into sections. Trans- 

 verse sections are the best to begin with. 



