38 



CYTOLOGY OF YEASTS 



that of other fungi cells with a perfectly characterized nucleus. 

 These investigations have shown that the body which Wagner took 

 for a nuclear vacuole is in reality another thing simply a vacuole 

 with metachromatic corpuscles. In contradistinction to the nucleus 

 of Wagner, it is not homogeneous. The existence of a nucleus can- 

 not be doubted. The presence of it is now admitted. 



Let us now investigate with detail the different elements which 

 make up the yeast cell, that is, the nucleus, cytoplasm, the elements 

 which it contains, and finally the cell membrane. Then let us 

 review the phenomena which evolution has accomplished in the celL 



The Nucleus 



The nucleus is relatively large in comparison to the cell (about 

 1 ju in diameter). It occupies a variable position, depending upon 



the form of the cell and its stage of develop- 

 ment. Whatever its location, it is often 

 closely associated with the vacuole which 

 encloses the metachromatic granules. This 

 is easily explained by the fact that the 

 nucleus seems to play a role in nutrition and 

 secretion, and that the vacuole is the seat of 

 an intense secretion. 



The nucleus almost always presents a 

 well-differentiated structure. It is surrounded 

 by a colored membrane, filled with a colorless 



Flg - , ^ 6 - ~7 Saccharomyces interior in which are a nucleolus and a chro- 

 ellipsoideus with Its 

 Nucleus. matic framework more or less abundant and 



From a Preparation stained visible, depending upon the species. (Fig. 



with Hematoxylin. . _ \ T .. Jl . , . . ,, 



46.) In S. cerevisiae this chromatic frame- 

 work is very distinct, the chromatin being particularly abundant. 

 By its structure this nucleus is not distinguishable from the nucleus 

 of other organisms, notably those which are present in most of the 

 fungi. Today the nucleus is unique even in those cells which are elon- 

 gated and which tend to form the rudiments of a mycelium. Later 

 on we shall take up nuclear division. 



The Cytoplasm and Its Different Products 



The cytoplasm undergoes, as will be seen in connection with the 

 evolution of cells, great variations in the course of development. 

 Very dense and homogeneous in young cells, it encloses in the ma- 

 jority of yeasts, especially the spherical or oval yeasts (S. cere- 

 visiae, ellipsoideus, Pastorianus, etc.), a vacuole filled with corpuscles 



