provide an oriented field in which forces inherent in the chromosomes 

 themselves contribute to their movement during anaphase. Electron 

 microscope studies carried out recently by Sato (1960) on cell division 

 in pollen mother cells of LiUum indicate that the chromosomal fibers of 



^l^i^T 



(b) 



Figure 6-8. Electron Micrographs Showing Formation of Chromosomal 

 Fiber During Meiosis in Pollen Mother Cells of Lily: (a) beginning of 

 development of chromosomal fiber in first meiotic division of Lilium lanci- 

 folium. Note granules drawn up in several lines from the kinetochore region 

 of the chromosome and extending toward the pole to form a bundle of 

 fibers. Within this bundle short fibrils are formed; 6500X. (b) Part of com- 

 plete chromosomal fiber in metaphase I of pollen mother cell of Lilium 

 speciosum. The chromosomal fiber is made up of a bundle of fibrils, about 

 15 in number; 6500X. (From Sato, S., 1960. "Electron Microscope Studies 

 on the Mitotic Figure. III. Process of the Spindle Formation," Cytologia, 25, 

 Figs. 6 and 7, p. 126. Courtesy of Dr. S. Sato. University of Tokyo.) 



the spindle are developed from the kinetochore of each chromosome and 

 grow toward the pole of the cell (Figure 6-8 (a) and (b)). The fact 

 that akinetic chromosome fraigments at metaphase do not undergo ana- 

 phase movement supports the view that there is an actual attachment of 

 the chromosomes to the spindle apparatus. Also, the abortive attempts 



MECHANICS AND PHYSIOLOGY OF CELL DIVISION / 151 



