THE CELL AND CELL DIVISION 49 



distinctly, toward the region of the centrosphere; in many 

 cells such a polarization of the linin toward the centrosome 

 exists throughout the vegetative phase. The two centrosomes 

 now begin to diverge and the surrounding centrosphere pulls 

 in two, one portion accompanying each centrosome (Figs. 20, E; 

 21, D). As the centrospheres diverge they enlarge, and within 

 each appear fibers radiating from the centrosome as a center 

 and producing the asters. ^Tiile the chromatic and achromatic 

 parts of the nucleus have been passing through these early 

 stages of mitosis, the nucleolus when present becomes vacuo- 

 lated, commences to dissolve and finally disappears. Soon 

 the nuclear membrane also commences to break down and 

 dissolve, first in the region of the asters, leaving the nuclear 

 substance free in the cytoplasm. Next the chromatin thread 

 shortens and thickens, breaking in the case of the continuous 

 spireme, into a number of separate segments or rods; or if the 

 spireme itself is of the segmented type, its elements now shorten 

 and thicken. When the spireme segments, the linin thread 

 upon which the chromatin granules are strung may remain 

 continuous between as well as through the chromatic rods. 

 These chromatic segments now become quite homogeneous, 

 clearly differentiated structures called the chromosomes (Figs. 

 20 C, E; 21, D; 22, C). Strictly speaking, each chromosome 

 consists of a dense mass of fused chromatin granules with a 

 portion of linin embedded. 



In practically all organisms in which the nucleus is a definitely 

 formed structure, the number of chromosomes appearing during 

 mitosis is fixed, and is constant throughout all divisions of 



nuclear membrane, ingrowth of the astral rays, segmentation of the spireme- 

 thread to form the chromosomes (examples, epidermal cells of salamander, forma- 

 tion of the polar bodies). E. Later prophase of type C; fading of the nuclear 

 membrane at the poles, formation of a new spindle inside the nucleus; precocious 

 splitting of the chromosomes (the latter not characteristic of this type alone). 

 F. The mitotic figure established. G. Metaphase; splitting of the chromo- 

 somes (e.p.) ; n, the cast-off nucleolus. H. Four stages in the divergence of the 

 two halves of a chromosome. /. Anaphase; the daughter-chromosomes diverg- 

 ing, between them the interzonal fibers (i.f.), or central spindle; centrosomes al- 

 ready doubled in anticipation of the ensuing division. J. Late anaphase or 

 telophase, showing division of the cell-body, mid-body at the equator of the 

 spindle and beginning reconstruction of the daughter-nuclei. K. Division 

 completed. 



