262 CARL C. UNDEGREN 



The Centrochromatin 



The centrochromatin is a basophilic, Feulgen-positive substance closely 

 attached to the basic centrosome (probably by an acid-base reaction). Some 

 portion of it is usually in contact with the nuclear vacuole. It is partitioned 

 between the cells following budding by a direct division controlled by two 

 tiny centrioles. In the resting cell it may assume a spherical form and cover 

 most of the centrosome. In division it is usually present in the form of a long 

 strand. The centrosome and centrochromatin have been identified with the 

 nucleus by several workers, but this view has been criticized by Lindegren 

 (1949), Lindegren and Rafalko (1950), and Rafalko and Lindegren (1951). 

 The filament often bends on itself to assume a V- or U-shape. In some 

 preparations it appears to be composed of numerous small particles, but this 

 is due to poor fixation and is especially prevalent in preparations fixed with 

 alkali. The view that the centrochromatin is a single filament external to 

 the centrosome is supported by a multitude of observations on well-fixed 

 cells. Centrochromatin is probably homologous to the heterochromatin of 

 higher forms differing only in being carried on the centrosome rather than 

 the chromosome. 



The Nuclear Membrane and the Chromosomes 



The nuclear vacuole contains the chromosomes and the nucleolus. The 

 chromosomes are partitioned between mother and bud vacuole in a precise 

 orderly manner without recourse to a spindle. The wall of the nuclear vacuole 

 does not break down at any time in the life cycle; it is a permanent cellular 

 structure. 



The Cell Membrane and the Cell Wall 



The cell membrane is a permanent cell structure. The cell wall appears to 

 be formed de novo in the spores, but it may depend on the cell wall sur- 

 rounding the ascus for its origin. 



BUDDING 



Figure 16.3-1 shows a cell in which the acidophilic centrosome attached 

 to the nuclear vacuole is surrounded by the darkly staining cytoplasm. A 

 band of basophilic centrochromatin is securely applied to the side of the 

 centrosome and is also in contact with the nuclear vacuole. Greater differen- 

 tiation often reveals a small centriole at each end of this band. The nuclear 

 contents are unstained. 



Figure 16.3-2 shows the first step in the process of budding. The centro- 

 some produces a small conical process which forces its way through the 

 cytoplasm and erupts into the new bud shown in Figure 16.3-3. 



Figure 16.3-4. The nuclear vacuole sends out a long, slender process which 

 follows the centrosome into the bud. Although the cell wall is not visible in 

 these preparations it must be assumed that the cell wall never ruptures but is 



