62 



PRINCIPLES OF ANIMAL BIOLOGY 



(Fig. 44). These lumps increase in size until they coalesce into a plate, 

 which forms a new wall dividing the cell into two. 



Amitosis. — Amitosis is a type of cell division which involves no com- 

 plicated visible mechanism. The word means, literally, not mitosis. 

 Many supposed examples of amitosis are merely distorted forms of 

 mitosis, the distortion being due either to faulty preparation or to natural 

 degenerative changes in the cells. Preparations of cells have in some 

 cases been so defective that cell division was at first regarded as amitotic, 

 but better technique revealed some of the features of mitosis. Also, in 

 certain degenerate animals it appears that the process itself has become 

 so modified that even the most perfect preparations of dividing cells 

 resemble amitotic division very closely. 



Fig. 45. — Amitotic division of the nuclei in the follicle cells of the cricket's ovary. 

 Various stages of nuclear division are shown. (From Conklin.) 



Confusion has arisen from the fact that the nucleus of a cell may 

 divide without any subsequent division of the cell body. This division 

 is often called amitosis of the nucleus, but it is not amitotic cell division. 

 Follicle cells in the cricket's ovary (Fig. 45) show nuclear division of 

 this sort. 



Genetic Significance of Mitosis. — The longitudinal duplication of the 

 chromosomos and the equal distribution of sister chromosomes to the 

 cells in division, to which attention has been called, has a greater sig- 

 nificance than has yet been indicated. The chromosomes contain the 

 units of heredity, which are called genes (Chapter 17). It is these genes, 

 more than anything else, which are arranged in longitudinal pattern in 

 the chromosomes. In the division of the chromosomes, the greatest 

 imi)ortance attaches to the duplication of the genes. The necessity of 

 distributing a complete set of cihromosomes to each cell rests on the 



