THE ACHROMATIC FIGURE, CYTOKINESIS, AND CELL WALL 179 



during the telophases. Instead of forming thickenings which become an 

 equatorial cell plate, the connecting fibers play relatively little part in 

 cytokinesis. One or more granules may be differentiated on the fibers 

 at the equatorial region, forming the so-called "mid-body," but the 

 actual division of the cell is brought about by the development of a 

 cleavage furrow, as will be described in the section on cytokinesis. 



Intranuclear Figures. In the above described cases of mitosis in 

 plants and animals the achromatic figure is derived mainly from the cyto- 

 plasmic region of the cell, the nuclear materials playing a relatively 

 minor part. In a number of forms, both among animals and plants (fungi, 

 for example), the spindle arises entirely in the nuclear region, forming 

 an intranuclear figure which may be completely established before the 



FIG. 61. 



A, B, anaphase and telophase of mitosis in ascus of Laboulbenia chcetophora. X 1350. 

 (After Faull, 1912.) (See also Figt 22.) C, intranuclear mitotic figure in oogonium of 

 Fucus. (After Yamanouchi, 1909.) 



nuclear membrane disappears. Cases are known in which the centro- 

 somes themselves are also intranuclear, but usually these bodies lie in 

 the cytoplasm against the nuclear membrane, so that although the spindle 

 portion of the figure is within the nucleus the asters lie in the cytoplasm. 



In the division of the nucleus in the ascus of an ascomycete, 1 to take 

 a single example, the process is as follows (Figs. 22; 61 A, B): The 

 centrosome, which in ascomycetes is often discoid in shape, lies against 

 the nuclear membrane. As mitosis begins an aster develops in the cyto- 

 plasm about the centrosome, and the latter divides to form two daughter 

 centrosomes. The central spindle, if formed at all, does not persist. 

 From each of the daughter centrosomes, which begin to move apart 

 along the nuclear membrane, a group of fibers extends into the nucleus 

 where the chromosomes are being formed from the reticulum. The 

 centrosomes finally reach opposite sides of the nucleus, and their two 



1 For references to the literature of mitosis in ascomycetes see page 290. 



