COLLECTION, EXAMINATION, ETC. 319 



down to the primordial, or centre, chamber. Then 

 dry the section and re-warm the balsam, and with 

 the needle (mider a lens or low-power objective) turn 

 the test over, in the balsam, on to its now flattened 

 surface ; by transmitted light under the microscope 

 you can see in the centre of the section the little 

 round primordial chamber, and by focussing tlirougli 

 it you can judge how much must be rubbed away on 

 either side to reach the centre of this chamber, at 

 which point the section is to be taken. 



' When the test is worn down to this central 

 point on either side the section is once more turned 

 over, and, with a fine scalpel, the superfluous balsam 

 is scraped away round the section, after which the 

 rubbing down is carried to the finest possible degree. 

 This turning over must be accomplished with the 

 greatest care, for if the warmed balsam is too 

 "tacky" at this point the central chambers may be 

 carried away, or the outer ones (especially in the 

 case of the MilioUiicp) may disintegrate. To obviate 

 this the balsam must be fhoroiifjlihj fluidified, when the 

 section may be turned over by pushing one or two 

 hairs of a fine brush underneath it. In some species 

 the arrangement of the chambers is such that, when 

 the rubbing has been carried down to the central 

 chamber, the centre of the section finds itself 

 isolated from the periphery, and then, the turning 

 must be done in the cold or hardened state ; that is 

 to say, the whole mass of balsam, with the em- 

 bedded section, must be dexterously split off the 

 glass with the scalpel, and not warmed again until it 



