I 



GENERAL WORKING CONDITIONS, 

 PRECAUTIONS AND PREREQUISITES 



The first rule to be observed by the experimental embryolo- 

 glst is that of scrupulous cleanliness. Contamination in all 

 forms must be avoided. It represents a serious source of error. 

 Be at great pains to insure the absolute cleanliness of every utensil 

 used. This precaution is as important as any taken against 

 accidental contamination during the actual experimental work. 



Glassware 



Ordinarily one uses the common cleaning fluid, potassium 

 bichromate and sulphuric acid. Because of its great toxicity, 

 the least trace remaining on the glassware is deleterious to 

 living cells. Whenever it has been used, it must be thoroughly 

 removed, preferably by a prolonged washing in running water 

 with a final rinsing in distilled water. The same precaution 

 must be taken with nitric acid. After its use, the glassware 

 should be washed in running water until the most delicate test 

 for acid gives no reaction. Soap and soap powders, if not 

 thoroughly removed, give, when the glassware is used for 

 sea-water, a precipitate which adhering to the vessel will modify 

 the experimental results. I find that Bon Ami is superior to 

 soaps and soap powders because more readily removed. It 

 leaves the glass dishes bright and clear. My practice is to 

 scrub the dishes vigorously with Bon Ami two or three times, 

 rinsing after each scrubbing, then finally to apply a suspension 

 of the Bon Ami to the glassware, setting it aside until thoroughly 

 dried. Then the dried powder is removed with dried towels, 

 after which the glassware is rinsed in running water for several 

 minutes. The glassware is then stacked upright, and never 

 upside down for fear of chance contamination, on dry linen towels 



