38 Animal Micrology 



paper. Fold the narrow side margins of the paper up along the sides of 

 the block first, then do likewise with the ends of the paper. Turn the 

 ears which have been formed at each corner back over what is to be the 

 end of the box, and then fold the long end of the paper back to hold 

 the ears in place, and also to make the end of the box of the same height 

 as the sides. Manifestly, any size of box may be made by varying the 

 size of the block. With a little practice, the same kind of box may be 

 folded without the use of a wooden block. 



7. With a warm, wide-mouthed pipette transfer sufficient 

 melted paraffin to a paper box to cover the bottom, then, with 

 warm forceps, remove the tissue to the box. Next, fill the box 

 with melted paraffin. Orient the object with heated needles if 

 necessary. As soon as the paraffin has congealed sufficiently for 

 the surface to become opaque, cool it rapidly by plunging it into 

 cold water; otherwise, the paraffin will crystallize and become 

 unsuited for sectioning. 



Cautions. a) Tissues must be oriented (i. e., placed in proper 

 position for cutting) while the paraffin is still in liquid condition. 

 Arrange the tissue so that it will be cut at right angles (trans- 

 verse) or parallel to the surface of the organ. Avoid oblique 

 sections as they are very puzzling. For present purposes of 

 practice cut transverse sections. 



6) If whitish-looking patches are present in the block after 

 imbedding they are due to xylol which has been carried over into 

 the paraffin. If they occur in the immediate vicinity of the ob- 

 ject, the block should be placed in the bath again until melted, 

 and the object be reimbedded. 



c) Be sure that every piece of tissue is marked after it is im- 

 bedded. Tissues are sometimes kept in paraffin for months or 

 even years before they are finally sectioned. To mark, scratch 

 the number of the record card in the paraffin, or better, write it 

 on the paper box and leave the box in place. 



CUTTING SECTIONS 



8. Study the paraffin microtome (e. g., Fig. 28); identify the 

 parts and learn how the thickness of sections is controlled. 



9. Proceed with the block of paraffin containing the intestine. 

 Make it fast to the carrying disk of the microtome in the follow- 



