5 2 CELL-DI VISION 



marked degree from the other spindle-fibres ; and they are believed 

 by many observers to have an entirely different origin and function. 

 A view now widely held is that of Hermann, who regards these fibres 

 as belonging to a central spindle, surrounded by a peripheral layer 

 of mantle-fibres to which the chromosomes are attached, and only 

 exposed to view as the chromosomes separate. 1 They are sometimes 

 thickened in the equatorial region to form a body known as the cell- 

 plate or mid-body, which, in the case of plant-cells, takes part in the 

 formation of the membrane by which the daughter-cells are separated. 



4. Telophases. In the final phases of mitosis, the entire cell 

 divides in two in a plane passing through the equator of the spindle, 

 each of the daughter-cells receiving a group of chromosomes, half 

 of the spindle, and one of the asters with its centrosome. Meanwhile, 

 a daughter-nucleus is reconstructed in each cell from the group of 

 chromosomes it contains. The nature of this process differs greatly 

 in different kinds of cells. Sometimes, as in the epithelial cells of 

 amphibia, especially studied by Flemming and Rabl, and in many 

 plant-cells, the daughter-chromosomes become thickened, contorted, 

 and closely crowded to form a daughter-spireme, closely similar to that 

 of the mother-nucleus (Fig. 23); this becomes surrounded by a mem- 

 brane, the threads give forth branches, and thus produce a reticular 

 nucleus. A somewhat similar set of changes takes place in the seg- 

 menting eggs of Ascaris (Van Beneden, Boveri). In other cases, as 

 in many segmenting ova, each chromosome gives rise to a hollow 

 vesicle, after which the vesicles fuse together to produce a single 

 nucleus (Fig. 37). When first formed, the daughter-nuclei are of 

 equal size. If, however, division of the cell-body has been unequal, 

 the nuclei become, in the end, correspondingly unequal a fact 

 which, as Conklin and others have pointed out, proves that the size 

 of the nucleus is controlled by that of the cytoplasmic mass in which 

 it lies. 



The fate of the achromatic structures varies considerably, and has 

 been accurately determined in only a few cases. As a rule, the 

 spindle-fibres disappear more or less completely, but a portion of their 

 substance sometimes persists in a modified form. In dividing plant- 

 cells, the interzonal fibres become thickened at the equator of the 

 spindle and form a transverse plate of granules, known as the cell- 

 plate (Fig. 25), which gives rise to the membrane by which the two 

 daughter-cells are separated. The remainder of the spindle disap- 

 pears. A similar cell-plate occurs in some animal cells; but it is 

 often greatly reduced, and may form only a minute body known as 

 the mid-body (Zwischenkorper), which lies between the two cells after 



1 Cf. P . 74. 



