barber: heredity in certain micro-organisms. 45 



After the isolations have been made the cover is sealed over 

 a hollow slide by means of vaseline. I prefer slides with 

 shallow hollows for this, and find that if the hollows are 

 breathed on just before sealing the cover, sufficient moisture 

 will condense on the slide to keep the culture in a saturated 

 atmosphere. 



After the cover is safely sealed on the hollow slide the iso- 

 lations may be examined with higher powers and better light, 

 and any droplets marked so that they may easily be found at 

 later examination. To accomplish this, I generally dip a 

 very fine capillary rod in Brunswick black and with it make 

 one or more dots or lines on the cover above the droplet to be 

 marked. This is easily done under the two-thirds objective 

 after a little practice. It is very helpful to make a large 

 cross with Brunswick black or India ink on the cover before 

 isolating, since the lines serve as a guide for locating drop- 

 lets. Since the apparatus allows one to control the position 

 of the droplets accurately, they may be arranged thus : 



A B C D 



10 



2 



3 



4 



and any droplet recorded by letter and number. 



In putting away the culture to develop, one should, of 

 course, put it in a protected place, and avoid placing it so 

 that the cover will be exposed to a lower temperature than 

 the slide beneath, else drops of moisture may condense on the 

 cover and cause the droplets to run together. I use for the 

 most part a special slide with a shallow concavity much 

 larger than that of the ordinary type. 



I seldom meet with difficulty from the drying up of droplets 

 during isolation, if the filter-paper, lining the sides of the box, 

 is well wetted before beginning ; but, as an additional pre- 

 caution against both desiccation and contamination, one may 

 nearly close the open end of the box with wetted filter-paper 

 after the pipette has been introduced. If the organism iso- 

 lated is sensitive to the least concentration of the medium 

 through desiccation, it is well to add fresh sterile nutrient 

 fluid just before sealing. 



