39 6 APPENDIX. 



After thorough cooling and hardening, the frame is removed and the mass, 

 if still adherent to the glass plate, carefully loosened. Tissue properly em- 

 bedded in paraffin may be kept for years without deterioration, if guarded 

 against dust and excessive temperatures. Indeed, delicate objects, as em- 

 bryos, are much more satisfactorily preserved after being carefully embedded 

 than if kept for a long period in alcohol. It is often convenient and eco- 

 nomical to embed two or even more small objects at one time in the same 

 block, care being taken to mark, by some ineffaceable label, the character 

 and cutting planes of the enclosures. 



Several methods of double embedding have been devised with a view 

 of combining the advantages of celloidin with those of paraffin. For particular 

 lines of work they are valuable and fairly satisfactory. Their description 

 will be found in the special books on microscopical technique. 



Cutting Sections. Although thoroughly adequate sections may 

 often be obtained ' ' free-hand ' ' with a razor or suitable knife, for the most sat- 

 isfactory results an accurately constructed microtome is a necessity. If pos- 

 sible, a medium-sized instrument neither too small and light nor too large 

 and cumbersome should be chosen. As an all-round microtome, the type 

 of the Schanze leaves little to be desired, as it may be used with satisfac- 

 tion for all kinds of cutting, wet or dry, and is accurate and simple. If only 

 one knife is available, one of the Weigert form, with a straight slotted shank 

 at an angle, is to be preferred. If much ' ' ribbon-cutting " is to be done, a 

 special holder should be provided, by which the knife can be clamped at dif- 

 ferent points and successive portions of the blade be brought into use. For 

 serial sections, the Minot rotary microtome is most convenient. 



Celloidin sections are cut wet, both object and knife being kept con- 

 tinuously moistened with 80 alcohol. Preparatory to using the microtome, 

 it is advisable to remove the superfluous embedding material with a sharp 

 razor or knife, leaving a narrow zone, 1-2 mm. in width, surrounding the 

 object. One or more sections across the trimmed block are then made to 

 expose the surface to be sectioned. The fibre-block bearing the embedded 

 tissue is now securely clamped in the object holder and adjusted as to plane 

 of section and height. It is important not to raise the object too much since, 

 if the removal of a slice of too great thickness be attempted, the entire mass 

 of celloidin may be torn from the fibre-block. The desired level of the tissue 

 should be gained, therefore, by repeated thin sections, thereby giving the 

 opportunity to adjust the knife and object in the mutually most favorable 

 position. The knife is set at such an angle that, after striking the corner of 

 the object, it passes through the latter obliquely and completes the section 

 before coming to the end of the " track." The section is removed from the 

 blade and placed in a dish containing 80 alcohol. The knife is returned to 

 the beginning of its track, the object raised by the micrometer screw to the 

 desired thickness of the next section, and the blade again drawn through the 

 block with a steady unhesitating pull. 



Paraffin sections are cut dry. When the object is large and not 

 delicate, the paraffin block may be clamped directly in the jaws of the object 

 holder. If, however, the object is of small size and great delicacy, as an 

 embryo, the block of paraffin with the embedded tissue is fastened to one of 

 the metal ' ' tables ' ' supplied with the microtome. This is best accomplished 

 by covering the surface of the metal disk with a thin layer of melted paraffin, 

 on which the embedded object is firmly pressed, additional security being 

 given by running a heated knife or other convenient tool around the base of 

 the block. After thorough cooling, the block is ready for trimming prepar- 



