180 INTRODUCTION TO CYTOLOGY 



groups of fibers become arranged in the form of a sharp poled spindle 

 extending through the nucleus with the chromosomes at the equator. 

 The nuclear membrane commonly remains intact until the chromosomes 

 approach the poles at anaphase; it then disappears, allowing the 

 nucleolus, which has remained unchanged, to escape into the cytoplasm 

 nearby. Between the two densely packed daughter chromosome groups 

 there extends a long strand of chromatic material: this soon disappears 

 and the two daughter chromosome groups reorganize two daughter 

 nuclei not separated by a wall. In those cases in which the division of the 

 fungus nucleus is followed by the development of a separating wall the 

 latter is formed by a cleavage furrow independently of the achromatic 

 figure. 



Origin of the Figure. Having before us the above examples of the 

 achromatic figure, we may now refer very briefly to some of the ideas 

 which have been advanced regarding the details of its origin in the cell. 1 



Early observers looked upon the whole mitotic figure chromosomes, 

 spindle, and all as a transformed nucleus, all the structures being formed 

 from the nuclear material at each mitosis. Strasburger, who first held 

 this view, later (1888), with Hermann (1891), believed the spindle to arise 

 wholly from the cytoplasm, whereas O. Hertwig pointed out cases in 

 which the astral rays arise from the cytoplasm and the spindle from the 

 linin reticulum of the nucleus. Flemming (1891) derived the fibers from 

 the linin and the nuclear membrane. It soon became evident that the 

 spindle, although in some cases arising entirely within the nucleus or 

 wholly from the cytoplasm, is commonly made up of materials derived 

 from both regions, as is evident from the examples described in the fore- 

 going paragraphs. 



When van Beneden and Boveri announced their view that the centro- 

 some is a permanent cell organ, transmitted by division to daughter cells 

 and directly concerned in the formation of the asters, the theory was 

 adopted that the figure arises from the cytoplasm as a result of the 

 influence of the centrosome. The centrosome therefore came to be 

 known as "the dynamic center of the cell." Although this organ does 

 play a conspicuous role when present, its importance in connection with 

 the achromatic figure was somewhat diminished when it became evi- 

 dent that many centrosomes do not persist from one cell generation 

 to the next, and that such bodies are entirely absent from the cells of 

 higher plants. 



Rearrangement Theories. Many attempts have been made to account 

 for the formation of the achromatic fibrils in the cytoplasm. According 

 to some the fibers and astral rays arise as the result of a morphological 

 rearrangement. of the preexistent protoplasmic structure, chiefly under 



1 Extensive reviews of the early theories are given by Wilson (1900, pp. 72-86 and 

 316-329) 



