56 PRESERVING AND PREPARING PATHOLOGICAL SPECIMENS. 



when these are fixed to the slide aiid appropriately stained, the condi- 

 tions for the study of cytological details are more favorable than by any 

 other method. For the paraffin technique the specimen should be small, 

 say c.c. as a maximum limit. The specimen, freed from the preserva- 

 tive fluid, is dehydrated by graded alcohols as follows: thirty per cent, 

 fifty per cent, seventy per cent, eighty-five per cent, and finally ninety- 

 five per cent, remaining in each of these alcohols for two or several 

 hours, and is then placed in absolute alcohol. Complete dehydration of 

 the specimen is indispensable, for if the slightest trace of water be left 

 in the specimen, shrinkage or other artificial changes in the tissues are 

 apt to occur, when the specimen is transferred to the clearing media 

 preparatory to immersion in the melted paraffin. 



When the specimen is thoroughly dehydrated by absolute alcohol, it 

 may be put in xylol or oil of cedar, remaining in this until it sinks and 

 becomes clear. It is .then immersed in a small dish or glass box of 

 melted paraffin, kept in a constant temperature bath held at 52 C., 

 where it remains until completely permeated by the paraffin. It is best 

 to use paraffin which has a melting-point of 50 C. After the specimen 

 has remained for a while in the first dish of melted paraffin, it is trans- 

 ferred to a second and then to a third dish of the same, in order to re- 

 move any clearing oil remaining in the specimen. The length of time of 

 the paraffin immersion depends upon the size and density of the speci- 

 men; usually one-half hour is sufficient for small, soft, or porous frag- 

 ments; an hour or one and a half hours for larger and denser tissues. 

 A longer period of immersion may interfere with the finer structural 

 details of the tissues. 



A small paper box considerably larger than the specimen itself is 

 filled with melted paraffin, and with a warm needle or forceps the speci- 

 men is transferred to the paper box and set in its proper position in the 

 bottom, so that the surface to be cut lies against the bottom of the box. 

 In order to avoid the slow cooling of the paraffin around the specimen in 

 successive layers, which prevents the formation of a homogeneous mass, 

 the paper box with its contents is quickly cooled by being put into cold 

 water, even iced water. When the paraffin block is hard, it is fastened 

 with paraffin on to one of the various discs belonging to the paraffin 

 microtome, trimmed so as to have a rectangular cutting surface, and 

 sections are cut with a dry knife. In order to stain these sections, 

 the paraffin must be removed from the interstices; this maybe done with 

 xylol. But when the supporting paraffin is removed from the sections 

 they are liable to fall to pieces during the further staining and other 

 manipulations. The only practical plan therefore with the great major- 

 ity of paraffin sections is to affix them to a slide and carry them in this 

 way through the various staining and mounting procedures. 



The best way of affixing delicate paraffin sections to a slide is by 

 means of a thin film of albumen. Equal parts of white of egg and gly- 

 cerin are thoroughly stirred together, filtered through paper, and a 

 small amount of carbolic acid is added to prevent the growth of micro- 



