THE PHENOMENA OF LIFE. 8 



surrounding protoplasm into two parts as above described (direct divi- 

 sion), but is preceded by complicated changes in their nuclei (karyoki- 

 nesis). These changes consist in a gradual re-arrangement of the intra- 

 nuclear network of each nucleus (see p. 17), until two nuclei are formed 

 similar in all respects to the original one. The nucleus in a resting 

 condition, i.e., before any changes preceding division occur, consists of 

 a very close meshworkx>f fibrils, which stain deeply in carmine, embed- 

 ded in protoplasm, which does not possess this property, the whole 

 nucleus being contained in an envelope. The first change consists of a 

 slight enlargement, the disappearance of the envelope, and the increased 

 definition and thickness of the nuclear fibrils, which are also more sepa- 

 rated than they were, and stain better. This is the stage of convolution 

 (Fig. 7, B, c). The next step in the process is the arrangement of the 



FIG. 7. Karyokinesis. A, ordinary nucleus of a columnar epithelial cell ; B, c, the same nu- 

 cleus in the stage of convolution ; D. the wreath or rosette form ; s, the aster or single star ; p. a nu- 

 clear spin41e from the Descemet's endothelium of the frog's cornea ; o, H, i, diaster ; K, two daugh- 

 ter nuclei. (Klein.) 



fibrils into some definite figure by an alternate looping in and out around 

 a central space, by which means the rosette or lureath stage (Fig. 7, D) 

 is reached. The loops of the rosette next become divided at the peri- 

 phery, and their central points become more angular, so that the fibrils, 

 divided into portions of about equal length, are, as it were, doubled at 

 an acute angle, and radiate V-shaped from the centre, forming a star 

 (aster) or wheel (Fig. 7, B), or perhaps from two centres, in which case a 

 double star (diaster) results (Fig. 7, G, H, and i). After remaining 

 almost unchanged for some time, the V-shaped fibres being first re-ar- 

 ranged in the centre, side by side (angle outwards), tend to separate into 

 two bundles, which gradually assume position at either pole. From these 

 groups of fibrils the two nuclei of the new cells are formed (daughter 

 nuclei) (Fig. 7, K), and the changes they pass through before reaching 

 the resting condition are exactly those through which the^ original nu- 

 cleus (mother nucleus) has gone, but in a reverse order, viz., the star, 

 the rosette, and the convolution. During or shortly after the forma- 

 tion of the daughter nuclei the cell itself becomes constricted and then 

 divides in a line about midway between them. 



5. Decay and death of cells. There are two chief ways in which the 

 comparatively brief existence of cells is brought to an end: (1) Me- 

 chanical abrasion, (2) Chemical transformation. 



