CELLS IN DIVISION 



necessitates a divergent course of cytokinesis, yet, for example, a pre- 

 ceding period of enhanced cytoplasmic motion often seems common to 

 both. Circulatory streaming in the leaf cells of Elodea is sensitive to 

 hydrostatic pressure (Marsland'^^^) ; it would be of great interest to 

 study the division of plant cells under the influence of high pressures 

 from several points of view. Possibly something more would also be 

 learnt about the nature of the colloidal changes involved in the first 

 formation of the plant cell wall. 



MODIFICATIONS OF MITOSIS 



In this section will be discussed examples of variation in the normal 

 course of mitosis in cellular organisms in which the great majority of 

 somatic cell divisions elsewhere follow their usual course. We shall leave 

 out of account the special forms of nuclear division in the Protista, of 

 which those in Protozoa have been so admirably and fully described by 

 Belar.^^^ 



Changes in ploidy 



First we may consider modifications of somatic mitosis in which the 

 size or number of the chromosomes is increased or diminished. In the 

 former, duplication of chromosomal material is not followed by cell 

 division; the least modified instance of this is where an otherwise 



Figure 58 Nucleus of a cell from the testis 

 septum of Gerris lateralis X 1750. The 

 number of heterochromatic X chromo- 

 somes (16) indicate the degree of 

 heteroploidy. From Geitler^'^ {By 

 courtesy, Z- -^f/Z^o^cA.). 



normal nuclear division is not succeeded by cleavage. Binucleate cells 

 can be formed in this way in the regenerating mammalian liver (Beams 

 and King;^^^ Wilson and Ledug^^^). As such cells again enter mitosis, 

 the chromosomes of both nuclei join a common metaphase plate, and 

 after a normal anaphase and telophase, large daughter nuclei are 

 formed which are probably tetraploid; this process has been observed 

 in living spleen cells of the mouse in tissue culture by Fell and 

 HuGHES^^^ (Plate XI (15) ). Thus a tetraploid nucleus is formed in two 

 mitotic stages. A less indirect duphcation of chromosomes is seen in the 



149 



