304 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1925 



brought about with apparently the greatest ease, and where processes 

 of building up and breaking down are carried out in a manner of 

 which even to-day we have but very little conception. 



We will now turn for a moment to the consideration of yeast as 

 a living organism. 



The yeasts, as is well known, belong to the great family of the 

 fungi, and may be described as unicellular fungi, reproducing by 

 budding, and capable also of forming ascospores. This latter func- 

 tion is of importance from the point of view of classification, as 

 it serves to differentiate between what are regarded as the true 

 yeasts and certain other closely allied organisms, such as the torulas 

 and mycoderma. In the common process of budding, the bud, which 

 occurs tirst as a small protuberance on the surface of the cell, quickly 

 increases in size until it has attained roughly the dimensions of the 

 parent cell, after which it usually becomes detached, leading a sepa- 

 rate existence, and reproducing in turn by the same process. It 

 often happens that before the offspring cell has separated from the 

 parent cell it has itself connnenced to bud, and so chains or clusters 

 of connected cells may frequently be seen. 



In the second mode of i-eproduction to which reference is made 

 above, the yeast cell becomes changed into an asc, in which are 

 formed a number of spores which may vary from one to as many as 

 twelve, but is usually from two to four. The conditions which favor 

 this mode of reproduction arc the employment of young and. vigorous 

 cells, a moist surface, plenty of air, and a suitable temperature, usu- 

 ally about 25° C. Under these conditions, and at the end of about 

 ii4 hours, certain changes will be seen to be taking place in the pro- 

 toplasmic contents of many of the cells, especially the more vigor- 

 ous ones. The protoplasm becomes at first very granular, and then 

 signs of segregation become visible, the contents of the cell separat- 

 ing into a number of ill-defined portions, usually from 2 to -1, but in 

 some species as many as 8 or 12. A little later each of these segre- 

 gated portions of highly granular protoplasm becomes invested with 

 a membrane, and it can then be seen that the cell contains a number 

 of well-defined spores. During the formation and development ol 

 the spores the parent cell often swells considerably, and in the end 

 bursts, liberating the spores, each of which constitutes an individual 

 yeast cell, and is capable of reproducing in the ordinary way by 

 budding. It may be added that the line between budding and asco- 

 spore formation is not very sharp, and that it often happens that 

 budding and sporulation may be taking place simultaneously. As a 

 general rule the spores are spherical, but in some of the yeasts they 

 have very characteristic forms. It would seem that spore formation 



