REFERENCES 27 



acid molecules are held in positions from which they cannot 

 diffuse to a significant extent. If a fixing agent is applied to 

 such molecules once they are held in this way, they will be 

 permanently fixed in their normal position, apart from such 

 cracking and contraction as may occur. Artefacts due to crack- 

 ing and contraction are, of course, rather readily detected in a 

 specimen. 



In conclusion, Plates I and II illustrate two of the types of 

 artefacts which occur in chemical fixation. Plate I, Fig. A shows 

 the type of distribution of glycogen which is normally obtained 

 in liver with chemical fixation. Figure B shows the distribution 

 of glycogen obtained when freeze-drying is used as the fixing 

 technique. There is a striking contrast between the two results, 

 and it is evident that, as a result of the forces set up by diffusion 

 processes during the action of a chemical fixative, glycogen 

 in a cell is forced out of its normal position. Plate II, Fig. A 

 shows an ultraviolet photograph of rat Walker sarcoma fixed 

 in Camoy's fixative (acetic acid and alcohol) which is thought 

 by cytologists to be one of the best fixatives for the demonstra- 

 tion of nucleic acid in such material. In this preparation the 

 contrast in the resting nuclei is quite striking, and surrounding 

 the metaphase plate there is a clear zone containing practically 

 no material absorbing ultraviolet light. Plate II, Fig. B shows 

 quite different results obtained by freeze-drying. Contrast in 

 the resting nuclei is much less than in Plate II, Fig. A and there 

 is no clear zone surrounding the metaphase plate. It is obvious 

 that in the action of a chemical fixative considerable diffusion 

 may occur of a substance, probably ribonucleic acid, which ab- 

 sorbs light in the ultraviolet. After this demonstration, it seems 

 hardly necessary to emphasize the desirability of using the 

 freeze-drying technique whenever it is available. 



Plate III illustrates the quality of the results which may be 

 obtained by cytochemical methods subsequent to freeze-drying. 



REFERENCES 



Altmann. 1890. Die Elementarenorganismen und ihre Bezeihungen zu 

 den Zellen (von Veit, Leipzig). 



Bell. 1952. International Review of Cytology, 1, 35. 



Gersh. 1932. Anal. Rec, 58, 309. 



Gersh, Sylven, Sjostrand, and Bell. 1952. Freezing and Drying (Insti- 

 tute of Biology, London). 



