194 INTRODUCTION TO GENERAL PHYSIOLOGY 



in all cases. The fact shows that it is not to be assumed that the 

 minute structure of the protoplasm corresponds to that of the living 

 state. If, however, there are things to be seen in one kind of cell 

 that are not visible in another, we are justified in holding that some- 

 thing was present during life in one and not in the other. 



The most generally useful fixing solution appears to be that of 

 Bouin : 



Saturated solution of picric acid in water 60 c.c. 



Commercial formalin (40 per cent, formaldehyde) 18 



Glacial acetic acid 2 



The pieces of tissue should not be large, and may remain in the 

 mixture for one or two hours. Wash repeatedly in 70 per cent, 

 alcohol until no more yellow colour comes out. Transfer to 

 methylated spirit for forty-eight hours and then to chloroform. 



The material must be supported by being impregnated with and 

 embedded in paraffin, which should have a melting point of 50 . 

 Since paraffin is soluble in chloroform, the material may be trans- 

 ferred directly to melted paraffin, which should not be at a tempera- 

 ture higher than sufficient to keep it melted. The tissue remains 

 in this for one or more hours, according to size, and may be changed 

 to fresh paraffin, since the chloroform must be got rid off. In 

 the case of delicate tissues, it is better to pass through a solution 

 of paraffin in chloroform before placing in the pure paraffin. If 

 the piece of tissue is a rather thick one, it should be passed through 

 oil of cedar-wood between the spirit and paraffin, since chloroform 

 does not penetrate very well. 



A mould is made by wrapping paper around a wooden rod, 

 projecting beyond the end of the rod. The piece of tissue is taken 

 out of the paraffin, by means of warmed forceps, and placed in 

 position in the mould, so that the sections made transversely across 

 will be in the desired plane. The mould is filled with melted 

 paraffin and cooled as quickly as possible. The paper is taken off 

 and the cylinder separated from the wood. Paraffin may be sliced 

 off so as to leave the tissue at the apex of a pyramid. 



Sections are cut by fixing in a microtome. This is an instru- 

 ment by which the embedded tissue is advanced by fractions of a 

 millimetre at a time and slices cut off by a razor. The rocking 

 microtome of the Cambridge Instrument Co. is convenient. 

 Sections should be io/^ in thickness. They must next be mounted 

 on slides, and are usually stained in order to render their con- 

 stituents more easily visible. As they leave the microtome, they are 

 generally more or less folded or creased. To flatten them, pick up 

 carefully with forceps and lay on warm water (not above 40 ). 

 When flat, float them on to a microscope slide by bringing the 



