ii PKACTICAL HINTS 35 



cloves. The celloidin should then be cut as before so as to indicate 

 the position of the object, and the latter, in its celloidin block, 

 should be transferred to a mixture of cedar oil and hard paraffin 

 and heated to 56 for fifteen minutes, and then for fifteen minutes 

 to a bath of pure hard paraffin. 



This second method has the disadvantage of rendering the 

 embryonic tissues rather brittle, but one great advantage of 

 embedding in celloidin is that the tissues of the embryo become 

 penetrated while cold by a substance which hardens and gives them 

 support, before they are subjected to the ordeal of the hot paraffin 

 bath, which has a tendency to cause shrinkage. 



When the sections are cut they are best mounted by first 

 smearing the slide with Mayer's glycerine and albumen fixative, 

 then laying the sections upon it and adding a layer of water heated 

 to 60 ; the hot water cools at once to about 45, and this heat 

 will flatten out the sections without melting the paraffin. The water 

 is drained off and the slide dried on the top of the thermostat for 

 forty minutes. Then the paraffin can be melted off and washed off 

 in xylol. It should then be immersed in oil of cloves for one minute, 

 which softens and dissolves the celloidin. Then it should be placed, 

 not in absolute alcohol, but in 90 per cent alcohol, which washes off 

 the oil of cloves and at the same time removes the glycerine from the 

 glycerine and albumen fixative ; it finally coagulates the latter and also 

 hardens the semifluid celloidin, so that it forms an additional fixative 

 for securing the adhesion of the sections to the slide. Another 

 method is to transfer the slide from pure xylol to a mixture of equal 

 parts of xylol and absolute alcohol. The absolute alcohol coagulates 

 the albumen and removes the glycerine. The slide can then be 

 transferred to 90 per cent alcohol and thereafter to alcohol of lower 

 grades. Sections thus fixed will stand any further treatment without 

 becoming loose. 



Mayer's fixative is made by mixing white of egg strained through 

 muslin with an equal volume of glycerine. A few drops of thymol 

 are added to prevent the decomposition of the albumen. 



In staining objects which have been preserved in osmium 

 tetroxide it is often found that the black deposit of metallic osmium 

 in the tissues prevents the stain from taking effect. The best 

 general stain is Grenadier's (sometimes called Delafield's) haematoxylin. 

 This is best used in a solution made by diluting the concentrated 

 stain in three or four times its bulk of distilled water. This solution 

 should be filtered before being employed. If the sections be 

 previously immersed in a solution of borax -carmine in 70 per cent 

 alcohol it will be found that they can remain in it for 24 hours 

 without absorbing any stain, but if then they be transferred to the 

 solution of haematoxylin described above, they stain rapidly and 

 well. Excess of stain is removed by immersing the sections in a 

 solution of acid alcohol made by adding two drops of strong 

 hydrochloric acid to 100 c.c. of 70 per cent alcohol. If the 



