340 HORRY M. JONES 



edges of the Petri dish will fit loosely into this groove. Within 

 the circle described by this groove, about 2 cm. from an edge 

 of the slab; i.e., about 5 mm. from the inner edge of the groove, 

 drill a hole about 4 mm. in diameter and about half the thick- 

 ness of the slab in depth. Drill a similar hole for the opposite 

 side of the annular groove. Now drill horizontal holes from the 

 corresponding surface edges of the slab to meet the vertical 

 holes, and of such a diameter that small rubber stoppers may be 

 used to stopper these holes securely. With this slab as a base 

 for the Petri dish, the method of manipulation, provided it is 

 desired to grow all of the colonies on the surface of the media, 

 is somewhat as follows : Pour the agar or gelatin into the Petri 

 dish as usual and allow to solidify. Then flow suitable water 

 or broth dilutions of the material to be examined on the surface 



Petri Dish /Ocm. dia/vj. 





fuL ! '^ Tn — Jin 



Annular (Sroova 

 3mm Deepx 



-4 1 I I 



J \ \ Air Oufht 



C/'OS'S ^Section i^/ew of stone base 

 w/ttj petr/ djsh in po^i//on. 



of the medium. Tilt the dish to one side, and, with a sterile 

 pipette, withdraw the excess of the diluting fluid. Invert the 

 dish into its sterile cover and allow to drain. This draining 

 will prevent contaminations on the stone base from spreading 

 upward on the plate and so obviate the necessity of sterilizing 

 the base. Now place the dish, edges down, into the annular 

 groove of the slab, which has been previously warmed to a 

 temperature sufficient to melt paraffine with a melting point 

 of say 45 degrees Centigrade. The plate may be sealed to the 

 stone by flowing melted paraffine into the groove either before 

 or after the plate is put into position in the groove. The stone 

 is then allowed to cool until the paraffine is thoroughly con- 

 gealed. There are now, between the stone and the surface of 

 the medium, only about 20 cc. of air to be replaced by the inert 



