842 DIVISION OF CELLS. 



by a delicate reticular network, or plexus of fibrils, which has been called "chromatin," 

 " "nucleoplasm," "karyoplasma," and "karyomiton." The network stains readilywith pigments, 

 hence the name "chromatin" given to it by Flemming. The nodal points of the network 

 give a dotted or granular appearance to the nucleus, especially when it is examined with a low 

 power. The nuclear membrane also consists of chromatin (fig. 624). In the meshes of the net- 

 work iie nucleoli, which seem to differ in constitution, and perhaps in function. According to 

 Flemming, there are principal and accessory nucleoli in some nuclei. In Carnoy's nomenclature 

 the several parts are spoken of as a fine reticulum of fibrils, enclosing in its meshes a fluid the 

 enchyhma which contains various particles in suspension.] 



[Direct Cell-Division. A cell may divide directly, as it were, by simple cleavage, and in 

 the process the nucleus usually divides before the cell protoplasm. The nucleus becomes con- 

 stricted in the centre, has an hour-glass shape, and soon divides into two.] 



Fig. 625. 

 M itosis. A, nuclear reticulum, resting state ; B, preparing for division ; C, wreath stage ; D, 

 monaster stage ; E, barrel stage ; F, diaster stage ; G, daughter wreath stage; H, daughter 

 cells, passing to resting stage. 



[Indirect Cell-Division. Recent observations, confirmed by a great number of investi- 

 gators, conclusively prove that the process of division in cells is a very complicated one, the 

 changes in the nucleus being very remarkable. The terms karyokinesis, mitosis, or indirect 

 division have been applied to this process. Figs. 623, 625 show the changes that take place in 

 the nucleus. The chromatin or intranuclear network (a, B) passes into a convolution of 

 fibrils, while the nuclear envelope becomes less distinct, the fibrils at the same time becoming 

 thicker and forming loops, which gradually arrange themselves around a centre (c and d) in the 

 form of a wreath, rosette, or spirem (C). The fibres curve round both at the periphery and the 

 centre and form loops ; but wheu their peripheral connections are severed or dissolved, we 

 obtain a star-shaped form or aster (D), composed of single loops radiating from the centre (e). 

 The loops divide in the direction of their length ; their number is doubled, but they are 

 thinner. By this further subdivision, the whole is composed of fine radiating fibrils (/), which 

 gradually arrange themselves around two poles, or new centres, to form the barrel-form or 

 pithode (E) ; the two groups of loops then separate still further, and arrange themselves so as 

 to form a diaster, or double star (g), the two groups being separated by a substance called the 

 equatorial plate. Each of the groups of fibrils becomes more elongated, and forms a nuclear 

 spindle, which indicates the position of a new nucleus. The protoplasm separates into two 

 parte. In each of these parts the chromatin rearranges itself into an irregular coil, and the whole 

 is called dispirem (G), and when division is complete, the chromatin filaments assume the form 

 seen in a resting nucleus. This whole complex process may be accomplished in 1 to 4 hours. 

 The separate groups of fibrils again become convoluted, each group gets a nuclear membrane, 

 while the cell protoplasm divides, and two daughter nuclei are obtained from the original cell.] 



The following scheme represents some of the more important changes : 



Mother nucleus. Daughter nuclei. 



1. Network. 8. Network. 



2. Convolution. 7. Convolution. 



3. Wreath or Spirem. 6. Dispirem. 

 * A8 *er. 5. Diaster. 



Equatorial grouping of chromatin. 

 HI. Budding or gemmation occurs in a well-marked form among the polyps and in some 

 mfusorians ( Vorticella). A bud is given off by the parent, and gradually comes more and more 

 to resemble the latter. The bud either remains permanently attached to the parent, so that a 

 complex organism is produced, in which the digestive organs communicate with each other 

 directly, or in some cases there may be a " colony" with a common nervous system, such as the 



