THE ACHROMATIC FIGURE 151 



near the nuclei but continues to be evident for some time at the margin 

 of the growing cell-plate (Fig. 101). When a cell contains a large central 

 vacuole the parietal cytoplasm may form a broad strand through it, the 

 mitotic figure occupying this strand. In some cases the figure has been 

 reported to move across a broad cell as the cell-plate develops from one 

 side to the other. In all these examples, therefore, the achromatic figure 

 is not only concerned with nuclear division, but it is also involved in the 

 process of cytokinesis, which here follows very closely upon mitosis. 

 Cytokinesis and the development of the cell w^all about the cell-plate 

 will be described in the next chapter. 



In microsporocytes, which at the time of their division are usually 

 suspended more or less free from one another in sporangial fluid, the 

 development of the achromatic figure is essentially like that described 

 above, but owing to the shape and polarity of such cells the figure ordi- 

 narily presents a somewhat different aspect in its earlier stages. In Larix, 

 for example (Fig. 82), the cytoplasm when adequately fixed appears rather 

 homogeneous and contains many rod-shaped chondriosomes. During 

 the late prophase the chondriosomes move endwise toward the nucleus 

 and lie parallel with its membrane, forming a dense "perinuclear chon- 

 driosomal mantle." Improper fixation here gives the "radial" and 

 "felted" aspects so often described. The chondriosomal mantle remains 

 intact throughout mitosis, its inner boundary marking the limit of the 

 nuclear region. As the nuclear membrane shrinks and disappears, the 

 chromosomes become grouped at the center of the nucleus, whose periph- 

 eral region is then occupied by the intranuclear spindle substance. 

 The cytoplasm contributes no formed element to the figure. The "trac- 

 tile fibers" appear first at the chromosomes and develop centrifugally 

 until the completed spindle extends across the nuclear region with its 

 poles at the chondriosomal mantle and the remaining intranuclear sub- 

 stance surrounding it on the sides. The figure is bipolar from the begin- 

 ning; multipolar appearances and extra fibers surrounding the spindle 

 proper are held to be due to inadequate fixation. In the telophase the 

 terminal portions of the spindle and much of the remaining intranuclear 

 substance become two masses of hyaline fluid in which the daughter 

 nuclei are reconstituted; hence the hyaline substance of these nuclei is 

 continuous with that of the original nucleus. The chondriosomes form 

 mantles about the two nuclei (Devise, 1914, 1922). In the peculiar four- 

 lobed sporocytes of certain liverworts the developing spindle passes 

 through a definitely quadripolar stage. ^ 



Many variations of the processes described above are known in 

 somatic cells and sporocytes. Among these the most instructive are 

 certain cases in which there is little or no shrinkage of the nuclear 

 membrane before the spindle differentiates. Frequently in sporocytes 



2 Farmer (1894, 1895), B. M. Davis (1899, 1901), A. C. Moore (1903). 



