80 PRINCIPLES OF ANIMAL BIOLOGY 



tion of the achromatic figure. What causes these fibers (if they are 

 fibers) to behave in the definite and orderly fashion which may be observed 

 in mitosis cannot be stated. 



Active migration of the protoplasmic substances, due no doubt to 

 their chemical reactions, has been appealed to to explain the move- 

 ments of the chromosomes. Another view attributes these movements 

 to the enlargement and collapse of multitudes of vesicles in the proto- 

 plasm. Further knowledge of the mechanism of mitosis probably 

 awaits discoveries in the field of colloid chemistry. 



It was once thought that the whole process of mitosis was under 

 the control of the centrosome which was supposed to be present in every 

 cell. When it became known that the centrosomes behaved in a different 

 manner in various cells and that although they were non-existent in 

 most plant cells mitosis proceeded in an orderly fashion in such cells, 

 the idea of the importance of the centrosome was abandoned. The 

 splitting of the chromosomes is probably to be accounted for by some 

 form of protoplasmic movement. It is not impossible, also, that their 

 movement in the anaphases of mitosis is due, at least in large part, to 

 active migration of the chromosomes themselves. 



AMITOSIS 



In contrast to the exceedingly complicated method of cell division 

 outlined above is the method of direct cell division to which the name 

 amitosis is applied. The process is begun by an elongation of the nucle- 

 olus and a constriction of the nuclear membrane. The nucleolus finally 

 separates into two portions and the constriction about the nucleus con- 

 tinues until the nucleus is cut in two, each part of the nucleus contain- 

 ing one of the halves of the nucleolus. Figure 51 illustrates this mode of 

 division. Not infrequently a process occurs which resembles incomplete 

 amitosis, but which may not be strictly related to cell division at all. 

 Thus the nucleus may become and remain bilobed, but the cell not divide. 

 In certain cases, for example in the follicle cells of the cricket's ovary, the 

 nucleus completely divides in amitotic fashion, but the cytoplasm remains 

 undivided. In this instance amitotic nuclear division does not result in 

 cell division but in an increase of nuclear surface. Nuclear division of a 

 similar type has been reported in developing muscle cells and is said to be 

 common in stratified epithelial cells of higher vertebrates. According 

 to some investigators cell multiplication is carried on by mitosis usually, 

 but late in the series of cell generations nuclear amitosis occurs giving 

 increased nuclear surface without division of the cell; while observations 

 by at least one investigator have shown that in tissue cultures the nuclei 

 may divide at first by amitosis and later by mitosis. 



Limited Occurrence of Amitosis. — Amitosis as a mode of cell division 

 seems to be of limited occurrence. It has been reported by numerous 



