CELL DTVrSION 



81 



investigators in dividing cells of young animals, in reproductive tissues, 

 and in regeneration, but many of these instances are in question. Other 

 investigators who have examined reputed cases of amitosis are of the 

 opinion that the appearance of the nucleus or of the cell and nucleus 

 resembhng stagea in amitosis can be interpreted without referring them 

 to cell division at all. For these reasons it must be considered until 

 more is known concerning the process that amitosis is of little importance 

 and of limited occurrence. The relation between mitosis and amitosis 

 is very incompletely known. In some degenerate animals there are 

 intergradations between mitosis and amitosis, so that the latter appears 

 to be only a modified form of mitosis. 



Mitosis and Amitosis in the Protozoa. — Among Protozoa, both 

 amitotic and mitotic methods of nuclear division occur. In many 



Fig. 51. — Amitosis in the follicle cells ot the cricket's ovary. Various stages of nuclear 



division are shown. {Frorji Conklin.) 



instances, as in Coccidium schubergi, amitosis is the method of division 

 emploj^ed. In certain infusorians, as Paramecium, the macronucleus 

 divides by amitosis and the micronucleus divides by a primitive method 

 of mitosis. In Ama'ba diplomitotica two types of mitosis occur. In 

 one type the chromosomes are arranged in an equatorial plate while in 

 the other type they are not so arranged. An almost typical mitosis 

 occurs in the rhizopod Euglypha, except that the entire process occurs 

 within the nuclear membrane (Fig. 49), whereas in the mitoses which 

 occur in the cells of most metazoa the nuclear membrane disappears anii 

 cytoplasmic elements enter into the process. In some of the Heliozoa, 

 as 'in Actinasphaerium and Actinophrys, the nuclear membrane persists, 

 while in Anthocystis the nuclear membrane breaks down completely 

 and extra-nuclear elements cooperate in the process of mitosis. 



6 



