64 



THE STRUGGLE FOR EXISTENCE 



preparation. But on the whole this method enables one to have 

 sufficiently standardized conditions for cultivation. 



The nutritive medium was sterilized in a large flask and then asep- 

 tically poured into small vessels for cultivation. These vessels were 

 previously sterilized by dry heat (by heating to 180° for three hours). 

 This method has many advantages as compared with the direct 

 sterilization of the nutritive medium in small culture vessels. The 

 fact is that when a liquid is heated in glass vessels in an autoclave, 

 even if the best kind of glass be used, the latter can somewhat alter 

 the composition of the nutritive liquid. This produces a considerable 



/0cm 3 



/0cm 3 



«m*v 



Fig. 8. The vessels for cultivation of yeast: (a) test tube, (b) Erlenmeyer's 

 flask. 



variation in the initial conditions of separate microcosms. The ves- 

 sels used for cultivation belonged to two types: (1) in experiments 

 with the deficiency in oxygen we used common test tubes with a 

 diameter of 13 mm. Ten cm 3 of nutritive medium were poured into 

 such a tube, the depth of the liquid being about 80 mm. (2) To 

 obtain better aeration, cultures were made in small Erlenmeyer 

 flasks of about 50 mm in diameter, and when 10 cm 3 of nutritive 

 medium were poured in, the liquid reached a depth of 7-8 mm. In 

 these conditions the layer of the liquid was almost ten times thinner 

 than in the test tubes (Fig. 8). The test tubes as well as Erlenmeyer 



