THE MECHANISM OF MITOSIS lOI 



by him four years later in the following words : " In our opinion 

 all the internal movements that accompany cell-division have their 

 immediate cause in the contractility of the protoplasmic fibrillae and 

 their arrangement in a kind of radial muscular system, composed of 

 antagonizing groups " (/.r. the asters with their rays). " In this sys- 

 tem the central corpuscle (centrosome) plays the part of an organ of 

 insertion. It is the first of all the various organs of the cells to divide, 

 and its division leads to the grouping of the contractile elements in 

 two systems, each having its own centre. The presence of these two 

 systems brings about cell-division, and actively determines the paths 

 of the secondary chromatic asters" {i.e. the daughter-groups of chro- 

 mosomes) " in opposite directions. An important part of the phe- 

 nomena of (karyo-) kinesis has its efficient cause, not in the nucleus, 

 but in the protoplasmic body of the cell." ^ This beautiful hypothesis 

 was based on very convincing evidence derived from the study of the 

 Ascaris Qgg, and it was here that Van Beneden first demonstrated the 

 fact, already suspected by Flemming, that the daughter-chromosomes 

 move apart to the poles of the spindle and give rise to the two 

 respective daughter-nuclei.^ 



Van Beneden's general hypothesis was accepted in the following 

 year by Boveri ('88, 2), who contributed many important additional 

 facts in its support, though neither his observations nor those of later 

 investigators have sustained Van Beneden's account of the grouping 

 of the astral rays. Boveri showed in the clearest manner that, during 

 the fertilization of Ascaris, the astral rays become attached to the 

 chromosomes of the germ-nuclei ; that each comes into connection 

 with rays from both the asters ; that the chromosomes, at first irregu- 

 larly scattered in the egg, are drawn into a position of equilibrium in 

 the equator of the spindle by the shortening of these rays (Figs. 90, 

 147); and that the rays thicken as tliey sJiorten. He showed that as 

 the chromosome splits, each half is connected only with rays (spindle- 

 fibres) from the aster on its own side; and he followed, step by step, 

 the shortening and thickening of these rays as the daughter-chromo- 

 somes diverge. In all these operations the behaviour of the rays is 



1 '87, p. 280. 



2 '83, p. 544. Van Beneden describes the astral rays, both in Ascaris and in tunicates, as 

 differentiated into several groups. One set, forming the " principal cone," are attached to 

 the chromosomes and form one-half of the spindle, and, by the contractions of these fibres, 

 the chromosomes are passively dragged apart. An opposite group, forming the " antipodal 

 cone," extend from the centrosome to the cell-periphery, the base of the cone forming the 

 "polar circle." These rays, opposing the action of the principal cones, not only hold the 

 centrosomes in place, but, l)y their contractions, drag them apart, and thus cause an actual 

 divergence of the centres. The remaining astral rays are attached to the cell-periphery and 

 are limited by a subequatorial circle (Fig. 48). Later observations indicate, however, that 

 this arrangement of the astral rays is not of general occurrence, and that the rays often do 

 not reach the periphery, but lose themselves in the general reticulum. 



