176 



INTRODUCTION TO CYTOLOGY 



two sets of fibers are known respectively as mantle fibers and connecting 

 fibers. The latter are also collectively termed the central spindle. 



It is during the anaphases and telophases that the connecting fibers 

 become most evident; in mitotic figures with many chromosomes it may 

 be impossible to see them at metaphase. At the beginning of the telo- 

 phase they may form a bundle no greater in diameter than the daughter 

 chromosome groups, but as the daughter nuclei reorganize the fibers 

 commonly bend outward at the middle, forming a barrel-shaped phrag- 

 moplast (Fig. 58, C) which in plants usually continues to widen by the 

 addition of new fibers until it comes in contact with the lateral walls of 

 the cell. 



V 



A 





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Fig. 58. 



A, spindle beginning to differentiate in kinoplasmic caps at poles of nucleus in Nephro- 

 dium. (After Yamanouchi, 1908.) B, same in Marsilia. (After Berghs, 1909.) C, D, 

 the origin of the cell wall in Pinus: C, connecting fibers between daughter nuclei at telo- 

 phase; D, thickenings appearing on fibers. E, the continued extension of the cell wall 

 after the completion of mitosis in the endosperm of Physostegia virginiana. X 215. (After 

 Sharp, 1911.) F, multipolar stage of spindle development in microsporocyte of Acer 

 Negundo. X 1125. (After Taylor, 1920.) 



While the above changes are occurring the new cell wall which is to 

 be formed between the daughter nuclei begins to differentiate. As the 

 central spindle widens the fibers become fainter near the nuclei and more 

 prominent at the equatorial region: this appearance seems to be due to 

 the flow of the material composing the fibers toward the latter region. 

 On the thickened fibers there now appear small swellings (Fig. 58, D) 

 which increase in size until they fuse to form a continuous plate across 

 the equator of the mother cell, thus dividing the latter into two daughter 

 cells. As this cell plate undergoes further changes (see p. 190) the fibers 

 disappear completely, first near the two nuclei and ultimately at the 

 equatorial region near the new wall. If the cell undergoing division 

 is very broad it often happens that wall formation begins near the center 

 of the phragmoplast while the latter is still extending laterally. In 



