the spindle and its activity (Wilson, 1900). The centrosome is a highly 

 conspicuous structure in certain animal cells, notably during most matu- 

 ration divisions and early cleavage stages of embryogeny. Generally, 

 the centrosome consists of a small, dense, refractile body embedded in 

 a clearer area from which fibrillar rays emanate, especially at the onset 

 of division. The central body, usually called the centriole, is normally 

 positioned in the cytoplasm just outside the nuclear membrane and 



(a) (b) 



Figure 6-2. (a) First Maturation Division Spindle in Living Oocyte of 

 Chaetopterus pergamentaceous Photographed with Polarization Optics. The 

 bright chromosomal and continuous fibers of the spindle and the dark astral 

 rays are all positively birefrigent and appear as dark bodies at the equator 

 of the cell, (lb) Tracing of First Maturation Division Spindle of Chaetopterus 

 Showing Detail of Component Parts. (From Inoue, S., 1953. "Polarization 

 Optical Studies of the Mitotic Spindle. I. The Demonstration of Spindle 

 Fibers in Living Cells," Chromosoma, 5, Figs. 2 and 3, p. 490.) 



divides during the early part of mitosis. The products of division migrate 

 until they take up positions opposite each other with the nucleus be- 

 tween them. The so-called astral rays normally develop during the time 

 of this migration. What connection these bodies have in the formation 

 of the functional spindle is by no means clear despite many years of 

 study. Proponents of the many versions of the centrosomal theory of 

 spindle organization have been somewhat embarrassed by the finding 

 that no such apparatus can be located in the cells of higher plants and 

 that, furthermore, its existence is doubtful in many animal cells. In 

 plant cells generally, and in many animal cells where there is no ap- 

 parent centrosomal apparatus, no organized spindle is obvious until 



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