VIROLOGICAL METHODS 257 



increase or growth of a virus in that tissue because some viruses, in suffi- 

 cient concentration, are toxic even though there is Uttle or no evidence of 

 increase. 



Harvesting tissue. Chapter X discusses general procedures for 

 autopsy. Some departures from the general methods are peculiarly 

 adapted to the virologist. Some viruses are propagated in the lungs for 

 a supply of antigen. A considerable number of lungs are desired at times 

 if mice have been used. In the interest of speed considerable time may 

 be saved if the lungs are removed from the back as follows. The sacri- 

 ficed mouse is pinned to the autopsy board in a prone position. After 

 the dorsal fur has been wet with 70 per cent ethanol, the skin over the 

 backbone is pulled up \\ith forceps. A V-shaped cut is made with 

 scissors running from the middle of the backbone up to the neck. The 

 backbone is secured with forceps just distal to the bases of the lungs. 

 Three more cuts with the scissors expose the lungs: one to sever the 

 vertebral column, a second anterolaterally from the severed column to 

 the left axilla, a third to the right axilla. 



Obtaining spinal cord from mice may be tedious or easy depending on 

 the method employed. The mouse is secured as above. A skin flap is 

 removed from over the vertebral column. The entire column is removed 

 by severing it from ribs and muscle attachments. With the column held 

 vertically, a stiff wire of such size as to just slip through the canal is used 

 to rod out the cord. 



If the tissues are to be used for cutting sections, care must be exercised 

 to avoid damaging the tissues. Two common errors are (1) to submit 

 tissue crushed by forceps and (2) failure to fix the tissue before autolysis 

 has progressed to a significant degree. 



If a suspension or extract of the tissues is desired, they can usually be 

 disintegrated by grinding with mortar and pestle. Sterile powdered 

 glass or other suitable abrasive aids the grinding process where fibrous 

 tissues are encountered. The abrasive should be removed by low-speed 

 centrifugation before the suspension for inoculum is used. 



Cultivation in Chicken Embryos 



Supply Source. Embryonated eggs have become one of the most useful 

 culture media for virologists. While eggs from various fowl have been 

 used, chicken eggs, because of the availability and relatively low cost, are 

 most popular. As with animals the source of supply is important. Usu- 

 ally the embryonated eggs can be supplied by a local hatchery or by con- 

 tract with owners of farm flocks. Clean, white-shelled eggs of high 

 fertility are desirable. Usually the investigator has little control over 

 the feed used and cleanliness of quarters where the eggs are produced. 

 There is some reason to believe that the more or less regular use of anti- 



