THE BRYOZOA BASSLEK. 349 



ment is pinched from the specimen and is rubbed upon the sandstone 

 until the surface of which a section is desired is perfectly flat. This 

 surface is then smoothed upon the hone, after which it is cemented 

 upon a glass slip with Canada balsam. The heating of the glass 

 slip to harden the Canada balsam is the most important part of the 

 process, for if the balsam be allowed to boil too long on the heating 

 stage or over a lamp it will be brittle when cold and the fragment 

 will spring off; if too short a time, the section when thin will granu- 

 late. After heating and subsequent cooling, the balsam should be 

 tested for hardness, the correct degree being intermediate between 

 brittleness and the point where the finger nail can make an impres- 

 sion upon it. If too soft, the slip must be reheated ; but if too hard, 

 it is better to remove the fragment, clean it by smoothing it off on 

 the hone, and then reheating again. When of the proper hardness, 

 the glass slip is then placed in the excavation of the wooden block, 

 which is dipped into water to secure adhesion. Then after rubbing 

 away upon the sandstone or carborundum slab all of the superfluous 

 material until the section is quite thin, the slide is removed from the 

 block and the thinning of the section is completed upon the hone. 



In this process the glass slip becomes scratched and unsightly, so 

 for a permanent mount the entire slip should either be ground down 

 to give the ground-glass effect or, better still, the thin section should 

 be transferred to a clean slip and covered in the usual way for per- 

 manent preservation. The transfer is accomplished by first cleaning 

 off all old gum around the section with alcohol, then adding a drop 

 of fresh gum, heating, and when the thin section has become loosened 

 sliding it onto a clean glass slip with a sharp-pointed instrument. 



Specimens too small to be cut with the wire nippers are sectioned 

 by placing them on a slide in balsam which has been only partially 

 hardened by heating. They may then be rubbed clown until the 

 required surface of the section is reached. The balsam is then melted 

 and the specimens are turned over with a sharp-pointed instrument. 

 After cooling, the thin sections are made in the manner described 

 above. 



Although this method of sectioning applies particularly to the 

 Trcpostomata, it is employed to advantage in species of the other 

 orders where the zoarium is a solid mass composed of numerous tubes. 

 In all cases these sections must be prepared to show the peculiar 

 structural features of the bryozoa, particularly the inner immature 

 zone and the outer peripheral area, where the zooecia are in the mature 

 state and develop accessory features, such as acanthopores, mesopores, 

 diaphragms, etc. To observe these features two sections are always 

 needed, a vertical section parallel with the axis of growth of the 

 tube and a tangential section parallel to the surface and close enough 

 to it to show the structure of the mature zooecia. 



