OUTLINE OF INDIRECT DIVISION 



71 



fibres show during this period a series of deeply staining thickenings 

 in the equatorial plane forming the cell-plate or mid-body. In plant- 

 mitoses this is a very conspicuous structure ( Fig. 34). In animal cells 

 the mid-body is usually less developed and sometimes rudimentary, 

 being represented by only a few granules or even a single one 

 (Fig. 29). Its later history is described below. 



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 daugJiter-sph'enie , closely similar to that 

 of the mother-nucleus (Fig. 29); 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. 52). 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 i^e.g. the 

 Nebenkern, p. 163). In dividing plant-cells, the cell-plate finally 

 extends across the entire cell and splits into two layers, between 

 which appears the membrane by which the daughter-cells are cut 

 apart.i A nearly similar process occurs in a few animal cells,- but 

 as a rule the cell-plate is here greatly reduced and forms no mem- 

 brane, the cell dividing by constriction through the equatorial plane. 

 Even in this case, however, the division-plane is often indicated 

 before division takes place by a peculiar modification of the cyto- 

 plasm in the equatorial plane outside the spindle (Fig. 30)- This 

 region is sometimes called the cytoplasmic plate, in contradistinction 

 to the spindle-plate, or mid-body proper. In the proi)hases and meta- 

 1 Cf. Strasburger, '98. 2 Cf. Hoffmann, '98. 



