GENERAL ANATOMY. 83 



Nuclear Spindle and Centrosomes. In spite of nu- 

 merous recent researches some features of cell division 

 remain still undecided; e.g., the spindle-fibres are by many 

 investigators explained as formed from the protoplasm of 

 the egg (cytoplasm), by others as derived from the achro- 

 matic nuclear substance. But the following questions 

 especially demand more thorough study. At the poles of 

 the spindle there have been often found colorless small 

 rounded corpuscles or plates, the ccntrosomcs. They are 

 especially conspicuous during the cleavage of the egg of 

 the round-worm (A scar is) (Fig. 92, />), and continue to be 

 visible after the completion of the daughter-nuclei ; by a 

 new division they form for each daughter-nucleus two cen- 

 trosomes, which pass to opposite ends of the nucleus and 

 thereby introduce again the process of division. Since, 

 also, centrosomes have been found in resting vegetable and 

 animal nuclei, the theory has been proposed that, in addi- 

 tion to the nucleus, there is present permamently a special 

 cell-organ (the centrosome) for the inception of division. 



Multinuclearity, Multicellularity. Nuclear division 

 and cell division commonly constitute a well-arranged me- 

 chanical process, the separate phases of which are joined to 

 one another according to a definite law. The 

 plane of division stands perpendicular to the 

 long axis uniting the two poles of the spindle. 

 The poles of the spindle exert a determining 

 influence not only upon the chromosomes, 

 but also on the protoplasm of the cell ; the 

 latter arranges itself around the pole in ra- 

 diating lines, and generally leads to that 

 arrangement of the granules known as the 

 protoplasmic rays. Thus it comes that for FIG. 22 . Giant -ceil 



with many nuclei. 



each phase of nuclear division there is a cor- 

 responding definite phase of division of the cytoplasm. 

 But this interrelation of cytoplasm and nucleus is by no 

 means an unchangeable and indissoluble one, for very often 

 nuclear division takes place without participation of the 

 cytoplasm. If this process is repeated several times, there 



