Cultivation 



573 



dots, which gradually grow out to the surface, effect a slow lique- 

 faction of the medium, and then appear to be situated in little pits 

 with sloping sides. The appearance suggests that the plate is 

 full of Uttle holes or air-bubbles, and is due to the slow evaporation 

 of the liquefied gelatin. 



Under the microscope the colony of the cholera spirillum is 

 fairly well characterized. The little colonies that have not yet 

 reached the surface of the gelatin soon show a pale yellow color and 

 an irregular contour. They are coarsely granular, the largest 

 granules being in the center. As the colony increases in size the 

 granules do the same and attain a peculiar transparent appearance 

 suggestive of powdered glass. The slow liquefaction causes the 



Fig. 239. — Spirillum cholerse asiaticae; gelatin puncture cultures aged forty-eight 

 and sixty hours (Shakespeare). 



colony to be surrounded by a transparent halo. As the liquefied 

 gelatin evaporates, the colony begins to sink, and also to take on a 

 peculiar rosy color. 



Gelatin. — In puncture cultures in gelatin the growth is also 

 quite characteristic. It occurs along the entire puncture, but best 

 at the surface, where it is in contact with the atmosphere. Lique- 

 faction of the medium begins almost at once, keeps pace with 

 the growth, but is always more marked at the surface than lower 

 down. The result is the formation of a short, rather wide funnel 

 at the top of the puncture. As the growth continues, evapora- 

 tion of the medium takes place slowly, so that the liquefied gelatin 

 is lower than the surrounding solid portions, and the growth ap- 

 pears to be surmounted by an air-bubble. 



