24 R. G. PERKINS 



surface of the plate, and the clock started, a complete-circuit 

 smear was made covering all time periods from, nothing to 

 twenty-four hours. By marking the bottom of the plate the 

 exact age of any given point could readily be determined. The 

 apparatus, as illustrated in the accompanying photographs, 

 which are self-explanatory, has worked successfully, but re- 

 quires certain precautions for its use. In the first place, unless 

 the apparatus is contained in a practically saturated atmosphere, 

 the plate will dry out. We therefore cover the machine with a 

 bell jar set in a water seal at the base. In the earlier work a 

 cotton swab inoculated with the culture was used, but a theo- 

 retical objection to this method was the possibility of growth in 

 the swab, which might interfere with the accurate morphological 

 time interpretations. Even so, it would be at least as accurate 

 as the method of direct inoculations, which places organisms of 

 all ages on the inoculated media. To obviate this possible criti- 

 cism, sterile water w4th a suspension of the organisms was 

 used, but this apparently took up sufficient nutriment from the 

 culture medium to have a similar result, as indicated by a heavier 

 growth in the later periods. We now use small flaps of sterile 

 tinfoil about 0.5 cm. in width, and find that the capillary at- 

 traction of the surface of the medium gives a good continuous 

 contact. By this means organisms are left at the margin of the 

 sliding foil and develop under uniform conditions at all parts 

 of the smear. Organisms, for instance, which have left the in- 

 oculating foil at a point half way round the plate are twelve 

 hours old, and so on. 



It is obvious that for satisfactory results the medium must 

 be of a proper consistency, and that its surface and that of the 

 swab^carrying table must be parallel. With these precautions 

 it is easy to obtain an even smear with separate colonies along 

 the track of the swab. At the end of the twenty-four hours one 

 can remove the plate, and by the use of a series of sterile cover 

 slips get impressions of the entire circle, which may readily be 

 labeled so that any given hour of growth can be studied with 

 considerable accuracy. If it is desired to study cultures twenty- 

 four to forty-eight hours old, the plate can be removed from the 



