194 GENERAL PHYSIOLOGY 



consists of nucleins, arranges itself into threads which appear 

 loosely rolled up into a coil (Fig. 72, A). The threads, which 

 have given to this form of nuclear division the name of mitotic 

 division and have approximately equal lengths, split lengthwise so 

 that from each a double thread results. - At the same time the 

 nuclear membrane becomes dissolved, and at the two opposite 

 poles of the nuclear mass the centrosomes, or central bodies (p. 69), 

 surrounded by their protoplasmic radiations, now become visible, 

 the two being united to one another by a fibrous, spindle-shaped 

 figure which is derived from the achromatic substance mixed with 

 the protoplasm. The double threads form loops, and group 

 themselves in the equator of the achromatic nuclear spindle in 

 such a way that their angles are directed towards the centre 

 (Fig. 72, B). Presently the spindle-fibres, streaming out from the 

 centrosomes, by their own contraction divide the double threads in 

 such a way that one half of each is turned to- 

 ward one pole, the other half toward the other 

 (Fig. 72, G). Thus two groups of threads 

 separate from each other and from the 

 equator of the spindle (Fig. 72, Z>). With 

 this the progressive phase of nuclear 

 division is ended and the retrogressive phase 

 begins. The two groups of chromatic threads 

 proceed further and further toward the two 

 poles, so that the whole equatorial part of 

 the spindle becomes free (Fig. 72, E). 

 FIG. 73. Centrosomes with Presently the spindle-fibres between the 



protoplasmic radiation * -i , i -\ , 



m the division of the two groups begin to become indistinct, 



egg-ceii. (After Boveri.) and the t h rea ds become twisted again into 



a coil at each pole (Fig. 72, F). During 



this process the whole cell-body has become constricted by a 

 circular groove, the plane of which stands at right angles to the 

 axis of the two nuclear poles. The groove becomes deeper and 

 deeper, until finally the whole cell divides into two equal halves, 

 each of which possesses a nucleus ; the latter surrounds itself with 

 a new nuclear membrane, the spindle-fibres completely 

 disappearing, and thus returns to its resting-stage. Thus by the 

 division of the mother-cell two daughter-cells have arisen, and 

 these continue the growth on their own behalf (Fig. 72, F). 

 But during the division a phenomenon has appeared in the 

 protoplasm. Simultaneously with the appearance of the spindle, 

 the poles of which are formed by the centrosomes, two star- 

 shaped figures begin to appear in the protoplasm, by the latter 

 arranging itself at each pole like rays around the centrosome 

 as a centre ; the centrosomes thus become surrounded exactly 

 like two suns by a closed circle of rays (Fig. 73). As the spindle- 

 fibres become indistinct the protoplasmic rays also disappear. 



