6-21 THE YEAST CELL 



mechanism of coiling, producing six pairs. As soon as they are 

 completely coiled the six small pairs detach themselves from the 

 nuclear membrane. They vibrate in the vacuole in rapid Brownian 

 movement, and finally begin to orient themselves in a single com- 

 plex. This process can be observed in a series of steps in which 

 each one of the six pairs is at first distinctly identifiable but final- 

 ly becomes merged into a single structure, which appears to con- 

 sist only of two long paired bands stretching across the entire di- 

 ameter of the nuclear vacuole. Occasionally these bands are twist- 

 ed about each other. The chromosome complex is flat and ribbon- 

 shaped, much wider than it is thick. It has a pronounced longitu- 

 dinal split and the appearance of two flat ribbons attached to each 

 other by their edges. This flat characteristic becomes apparent 

 when the six pairs are being arranged in the complex. The next- 



i 



Fig. 6-15 The Central Structure is a Germinating Four-spored 

 Ascus. One ascospore is in sharp focus and the wall of the spore 

 can be seen enclosing the cell which consists of cytoplasm, crowd- 

 ed at one end, and the growing nuclear vacuole. Six haploid 

 chromosomes can be seen in the vacuole. Parts of two other 

 spores, one below and one to the lower left are visible; the 

 fourth is not visible in the optical section. The stain is tolui- 

 dine blue . 



step is the longitudinal splitting of the entire complex along the flat 

 plane. Partially split complexes, resembling Y-shaped figures, are 

 often visible. It is also possible to find a pair of complexes in the 

 cell, one lying across the other, each one extending the full distance 

 of the vacuole, often bending when they meet the inner surface. One 

 of these complexes passes into the bud vacuole by slipping through 

 the bud canal; and immediately afterwards, or during this passage, 

 begins to break down into twelve individual chromosomes. 



