CONDITIONS OF CELL REPRODUCTION. 225 



of sealing-wax (c) makes a tight joint over the stopper of rolled 

 asbestos-paper (d). The plug of cotton (b) in the neck of the flask 

 prevents over-rapid and excessive desiccation of the contained 

 cells, and at the same time excludes any extraneous germs that 

 may have slipped through the plug (a) in the cap tube. This 

 method of storage on cotton is of special value, and in fact indis- 

 pensable, when perfectly pure cultures are to be sent to tropical 

 countries and will be exposed to high air temperatures for some 

 time en route. Streak cultures on gelatin or agar-agar, which 

 are occasionally used for transport over short distances in the 

 temperate zone, are out of the question in such cases. 



265. Conditions of Cell Reproduction. 



Of the two possible methods of cell reproduction in yeast, 

 namely, budding (the vegetable form) and ascosporulation, the 

 second may be disregarded so far as the practice of yeast culture is 

 concerned, on account of the greatly restricted increase it furnishes. 

 The method of budding, with which alone we shall now have to 

 deal, has already been treated of in 245, though only from the 

 standpoint of the morphologist, the chief point being the variability 

 in the form and aggregation of the cells resulting from vegetative 

 reproduction. In the present paragraph this information will 

 be supplemented by a review of the external conditions of cell 

 reproduction, the requisite constitution of the nutrient medium 

 having been already dealt with in the two previous chapters. 

 Before, however, entering upon a discussion of this question, it 

 will be necessary to treat of the limitation of three definitions 

 which here come into application. 



The term " reproductive capacity" (occasionally also known as 

 " coefficient of reproduction") relates to the total number of cells 

 in the yeast crop obtainable under any given conditions (i.e., in- 

 dependently of time) per unit of the original sowing. This unit 

 may be taken on the basis of weight, in which event a reproductive 

 capacity of 20, for instance, implies that 20 grms. of yeast crop 

 have been obtained for every i grrn. of yeast sown. The crop is 

 usually obtained in the form of a sediment (see vol. ii. p. 1 14), and is 

 weighed either in the pressed condition (with a variable content 

 of water according to the degree of pressure) or else after drying. 

 The latter method is the most reliable, especially in experiments 

 on the consumption of nutrient substances, provided as we 

 know (vol. ii. p. 179) is not usually the case the deposit consists 

 of yeast cells exclusively. For this reason statemen ts, based on 

 unit weight, relating to the ratio of increase are of merely low and 

 conditional value. Far greater comparative reliability attaches to 

 determinations made with the cell-counter (vol. i. p. 124), and 

 referred to the unit of cells in the sowing. 



" Reproductive power " (also spoken of as " the velocity of 



