IMBEDDING. 407 



microscopic sections at the temperature of the room, a grade with a lower 

 melting point should be used. The melted paraffin should be in a paraffin 

 bath or in a thermostat maintained at a temperature but slightly above the 

 melting point of the paraffin. The tissue should not remain in hot paraffin 

 longer than is required; it is generally left 2 hours in one cup and then 

 is transferred to another in which it remains for two hours longer. The 

 purpose of this transfer is to free the tissue from chloroform, most of 

 which remains in the first cup. 



The imbedding frame in which the paraffin is to be cooled, is a box the 

 bottom of which is made by a glass plate and the sides of which are of metal 

 in two L shaped pieces. By sliding the latter back and forth in relation to 

 one another, the size of the space which they enclose can be varied. Before 

 using the frame the inside surfaces of the metal pieces together with that 

 surface of the glass on which they rest are rubbed with glycerine and the 

 frame is warmed by placing it for a few minutes on the top of the paraffin 

 bath. Melted paraffin is then poured into it, and the tissue, removed from 

 the cup by means of a spatula, is added. It sinks to the bottom and may 

 be placed in any desired position by means of needles warm enough to 

 prevent the paraffin from solidifying over their surface. The paraffin is 

 then quickly cooled by lowering the frame into a basin of cold water so 

 that the latter surrounds the metal pieces. Water must not reach the upper 

 surface of the paraffin until it has solidified; then the frame is placed under 

 water and in a few 7 minutes the glass plate and metal pieces may be detached 

 from the solid paraffin. As soon as it is thoroughly cool it may be sec- 

 tioned. 



Before the imbedded object is attached to a block of vulcanized fiber, 

 superfluous paraffin is cut away leaving the tissue rising from a broad 

 base of paraffin and completely surrounded by a thin layer. The base 

 is placed upon a heated spatula which rests upon the fiber block. When 

 the paraffin has melted somewhat, the spatula is withdrawn and the base 

 is pressed down upon the block, to which it adheres securely when the par- 

 affin has solidified again. The fiber block is then clamped in a " precision " 

 microtome. If a rotary microtome is used the paraffin is attached to a metal 

 disc in place of a fiber block. Sections should be from 6 to 10 n thick, but 

 under favorable conditions they may be made 2 ti thick. If the paraffin on 

 both sides of the tissue is trimmed parallel with the knife blade, the succes- 

 sive sections adhere to one another by their edges forming ribbons. Thus 

 the sections may easily be kept in order. The first one cut is attached 

 to the upper left hand corner of the slide, and the others follow like lines 

 upon a printed page. Sections mounted in this way are called serial 

 sections. Paraffin sections, as they are taken from the microtome, are laid 



