ZOOLOGY AMD BOTANY, MICROSCOPY, ETC. 951 



serum, or other nutrient material required by one or other species of 

 Bacterium. The gelatine-medium thus prepared may be kept in a 

 tube and a cultivation thus carried on upon its surface, or (and this is 

 its principal use) it may be spread when liquid on a microscope 

 object-slide and allowed to cool. Then such a gelatine plate may be 

 inoculated by touching its surface with material containing the 

 Bacteria which it is desired to study. The plate is readily protected 

 from the access of accidental atmospheric germs, and maintained at 

 such temperature and degree of moisture (by a glass shade) as the 

 experimenter may desire. The main point of advantage is this, that 

 the point of inoculation on the surface of the gelatine can, owing to 

 its transparency, be readily examined with the highest powers of the 

 Microscope, and the growth of the Bacteria followed ; whilst further, 

 owing to the fact that the medium in which the growth takes place is 

 solid, no mixture of the diiferent kinds which may be present occurs, 

 but each Bacterium produces around it a little spherical nest of its 

 own kind. From these nests, with a sterilized needle-point, individuals 

 can be removed to start new pui-e cultivations. 



But it is obvious that, if the original point of inoculation was 

 very minute, there is no danger of any accidental contamination from 

 atmospheric germs, for these are not likely to fall on the identical 

 spot no bigger than the puncture of a needle's point, where the 

 experimental culture is going on. As a matter of fact, where they 

 fall on to the gelatine there they remain and grow, and fifty such 

 accidental spores may fall on to the gelatine plate without in the 

 least interfering with the purity of the experimental culture. 



There is yet further a very simple device which enables Dr. Koch 

 to use this gelatine surface as a means of " spacing " and dividing the 

 various species in a mixture of Bacteria. He dips a sterilized needle 

 into such a mixture, and then makes a long shallow streak with the 

 needle's point upon the surface of the gelatine. The Bacteria which 

 were adhering to the needle's point are in this way dropped at intervals 

 along the streak, some nearer, some further apart, but all ( with rare 

 exceptions) in such a way that their subsequent growth keeps clear 

 of that of a neighbour, and can, with the aid of a low power or even 

 without any Microscope, be visited by a sterilized needle-point, and 

 thus used to start on another gelatine plate a perfectly pure cultivation. 



" It is only by such monosporous cultivations," writes Professor 

 Lankester, " that we can arrive at solid conclusions in reference to the 

 forms and activities of the Bacteria, e. g. as to whether one form can 

 give rise to progeny of another form when its food and conditions of 

 growth are changed, and again, as to whether special fermentative 

 powers can be lost or acquired in the course of generations derived 

 from one parent germ, but subjected to different conditions as to food, 

 temperature, and oxygen. The method of gelatine cultivation devised 

 by Dr. Koch places the means of following out these inquiries in the 

 hands of every careful microscopist. Such methods as Lister's were 

 too troublesome and too difficult for general and widespread applica- 

 tion ; but now that monosporous cultivation of Bacteria has been 

 rendered a comparatively simple and certain afiair, we may expect 



