CELL MULTIPLICATION IQ 



The Centrosome and Attraction Sphere. In addition to the nucleus, 

 a minute spherical body called the centrosome is believed to be constantly 

 present in animal cells, though sometimes too small to be demonstrated. 

 The centrosome is smaller than the nucleus, close to which it lies, and exerts a 

 peculiar attraction for the protoplasmic filaments and granules in its vicinity, 

 so that it is surrounded by a zone of fine radiating fibrils, forming the attrac- 

 tion sphere or archoplasm. Some authorities assert that the centrosome 

 lies within the nucleus in the resting state, and passes into the cell proper only 

 in the earlier stages of cell division. The attraction sphere is most distinctly 

 seen in cells about to divide. It plays an important role in nuclear division, 

 but it is doubted if it gives the initial impulse to the process. 



Cell Multiplication. Cells increase in number by a process known 

 as cell division, of which the first act is nuclear division. In fact the nucleus 

 is the center of control of the cell mass in the process of division. Cell multi- 

 plication takes place by two recognized methods, direct or amitosis, in which 

 there is little disturbance of the nuclear network, and indirect or mitosis, in 

 which there is a complex series of nuclear network changes. 



a. 



FIG. 13. Akinesis, Amitosis, or Direct Cell Division. A, Constriction of nucleus; B, 

 division of nucleus and constriction of cell body; C, daughter nuclei still connected by a 

 thread, division being delayed; D, division of cell body nearly complete. (After Arnold.) 



Direct Cell Division or Amitosis. The division of a cell is preceded 

 by division of its nucleus. Direct or simple division, amitosis or akinesis, see 

 figure 13, occurs without any change in the arrangement of the intranuclear 

 network. A constriction develops at the center of the nucleus, possibly pre- 

 ceded by division of the nucleoli, and gradually divides it into two equal 

 daughter nuclei. A similar constriction of the protoplasm of the cell occurs 

 between the daughter nuclei and divides it into two parts. 



Indirect Cell Division or Mitosis. Indirect division, mitosis, orkaryo- 

 kinesis is the usual method of cell division, and consists of a series of changes 

 in the arrangement of the intranuclear network, resulting in the exact division 



