NICK LES AND BASSLK.R.] MAKING OF SECTIONS. 123 



smoothed upon the hone, and the fragment is read}^ for mountinj^. A 

 drop of Canada ])al8am is placed upon the center of a glass slip of the 

 usual size, which, for economy's sake, may be cut from ordinary Avin- 

 dow glass, and the smoothed face of the fragmiMit upon it. The slip 

 is now heated on a heating stage or over a lamp and the Canada bal- 

 sam allowed to ])oil for a certain time, the length of which must be 

 learned by experience. The thickness of the glass and the amount of 

 gum are factors to be considered, but in general it may be stated that 

 with a slide of average thickness and a medium amount of gum the 

 boiling is complete when the edges of the balsam commence to turn 

 brown. The slip is then laid upon a horizontal piece of wood to cool. 

 After it is cold the balsam should be tested. The exact hardness 

 required must be intermediate between brittleness and the point where 

 the linger nail can make an impression upon it. If too soft, the slip 

 must be carefully reheated; if too hard, fresh gum and reheating maj^ 

 suffice, l)ut it is better to remove the fragment, clean it and the slide, 

 and remount. If of the proper hardness, the slip is placed in the exca- 

 vation of the block, M'hich has been dipped into water to secure adhesion, 

 and the superfluous material is rubbed away upon the sandstone. 

 When nearly thin enough the slide is taken out of the block and lin- 

 ished upon the hone. As the glass slip has become scratched and 

 generally unsightly during the foregoing process, the section ma}^ be 

 transferred to a good slip, mounted, and covered in the usual way for 

 permanent preservation. The slide should be carefulh^ labeled, so 

 that it can always be known from which particular specimen the sec- 

 tion was made. 



Specimens too small to be worked in the manner above described — 

 as, for example, small forms of Bythopora, Rhombopora, or Streblo- 

 trypa — may be sectioned in the following way: Place a little balsam 

 on a slip and heat it, but only enough to partially harden it. Into 

 this heated balsam place several specimens without any rubbing. 

 After cooling rul) down a little carefull}'; reheat cautiously, and with 

 a sharp-pointed instrument turn one or more specimens so that the 

 smoothed face is against the glass. Now cool and rub down farther. 

 Again cautioush" reheat, and turn the specimens which have not vet 

 been turned. After cooling rub again until the sections are thin 

 enough for microscopic examination, and finish as before. 



These sections must be prepared with a knowledge of certain struc- 

 tural features. The zoarium of most bryozoa is composed of two 

 zones, an inner, in which the zo(Bcia are immature, and an outer or 

 peripheral, in which the zocecia are in the mature state, and accessory 

 features, such as acanthopores and mesopores, are developed. To 

 observe all these features usualh" three sections are needed, a vertical, 

 or, in ramose or frondescent forms, a longitudinal parallel with the 

 axis, a transverse, which cuts across the axis, and a tangential, which 



