502 ir. M. CARLETON 



(if any) caused by niakinj^ up mercuric chloride or forniol in 

 distilled water or in isotonic saline. 



Technique. 



In these experiments every attempt was made to modify 

 only the tonicity of the sodium chloride and that alone. To 

 attain this end the following precautions were observed : 



(1) The concentration of the substance employed as a 

 fixing reagent was kept constant throughout the series, that 

 of the mercuric chloride being G per cent., that of the formol 

 5 per cent. The solutions of the latter were all prepared from 

 the same sample of conmiercial (40 per cent.) formol, since the 

 strength of this substance is very liable to variation, especially 

 after keeping. 15y 5 per cent, formol is meant a 5 per cent, 

 solution of formaldehyde, i.e. a solution prepared by diluting 

 one volume of 40 per cent, formol with seven volumes of 

 water. 



(2) The same volume of fixative (50 cc.) was always used. 



(3) Care was taken in cutting out the pieces of tissue for 

 fixation to keep them, so far as was possible, of the same size. 



(4) The conditions of embedding and staining were kept 

 constant. The tissues were dehydrated in ascending grades 

 of alcohol — beginning with 50 per cent, alcohol — cleared, and 

 embedded in paraflfin all in exactly the same manner. To 

 reduce cell-shrinkage during these processes to a minimum, 

 the tissues were passed from absolute alcohol to a mixture of 

 equal parts of absolute alcohol and xylol. They were then 

 transferred to pure xx\o\, next to xylol-paraffin, and finally 

 to pure paraffin. Further, all tissues were cut at the same 

 thickness (8 fi), and, as a final precaution, the sections used for 

 microscopic observation were taken after 50 cm. of the ribbon 

 containing the sections had been cut on the microtome. In 

 this way the sections were all taken from approximately the 

 same depth beneath the surface of the piece of tissue — a point 

 of some importance in view of the tendency of many fixatives 

 to shrink the periphery more than the centre of tissues. Sections 

 were stained on the slide with Ehrlich's haematoxvlin and 



