CULTURES, AND THEIR STUDY. 127 



being made in the culture-medium, in order that the 

 growth be symmetrical) that it is next to impossible to 

 make a microscopical examination of it with any power 

 beyond that given by a hand-lens. 



Much attention has been given of late to the preparation 

 of microtome sections of the gelatin growth. To accom- 

 plish this the glass is warmed sufficiently to allow the 

 gelatin to be removed and placed in Miiller's fluid (bi- 

 chromate of potassium 2. -2. 5, sulphate of sodium i, 

 water 100), where it is hardened. When quite firm it 

 is washed in water, passed through alcohols ascending 

 in strength from 50 to 100 per cent., imbedded in cel- 

 loidin, cut wet, and stained like a section of tissue. 



A ready method of doing this has been suggested by 

 Winkler, who bores a hole in a block of paraffin with 

 the smallest-size cork-borer, soaks the block in bichlorid 

 solution for an hour, pours liquid gelatin into the cavity, 

 allows it to solidify, inoculates it by the customary punc- 

 ture of the platinum wire, allows it to develop sufficiently, 

 and when ready cuts the sections under alcohol, subse- 

 quently staining them with much-diluted carbol-fuchsin. 



Very pretty museum specimens of plate- and puncture- 

 cultures in gelatin can be made by simultaneously killing 

 the micro-organisms and permanently fixing the gelatin 

 with formalin, which can either be sprayed upon the 

 gelatin or applied in dilute solution. As gelatin fixed 

 in formalin cannot subsequently be liquefied, such prep- 

 arations will last indefinitely. 



The growths which occur upon agar-agar are in many 

 ways less characteristic than those in gelatin, but as this 

 medium does not liquefy except at a high temperature 

 (100 C.), it has that great advantage over gelatin. The 

 colorless or almost colorless condition of the preparation 

 also aids in the detection of such chromogenesis as may 

 be the result of the micro-organismal growth. 



Sometimes the growth is colored, sometimes not ; some- 

 times the production of a soluble pigment colors the 

 agar-agar as well as the growth ; sometimes the growth 



