ENDOMITOSIS 



Duplication of chromosomes without cell or nuclear division in the 

 usual sense appears to be characteristic of some tissues in both plants 

 and animals. Such duplication gives rise to a state known as polyteny, 

 which means only that the chromosomes are obviously multistranded. 

 The widespread occurrence of this phenomenon has led to the notion 

 that a chromosome is potentially multistranded and that variations in 

 the degree may somehow be bound up with the question of cellular 

 differentiation (Huskins, 1947) and senescence. The general thesis that 

 variation in the number of ultimate or potential strands may be related 

 to nuclear metabolism would appear to warrant extensive consideration. 



VARIATIONS IN MITOSIS 



Variations in the mitotic process may be induced by a variety of 

 chemical and physical stimuli. In addition, very similar deviations from 

 the norm occur sporadically and spontaneously and are also character- 

 istic of the cells of some tissues. Berger (1938) and Grell (1946) 

 described a series of "reduction" divisions in certain gut cells of the 

 larva mosquito in which chromosome replication took place to a very 

 high degree and was followed by a rapid series of mitoses with no 

 further replication until the chromosomes again reached the normal 

 diploid strand level. Multiplication of chromosome strands without 

 mitosis appears to be rather common, especially in nuclei of differenti- 

 ated cells, as first noted by Geitler (1939) in the water skater, Gen is 

 lateralis. Later, Geitler (1941) pubHshed an extensive review of the 

 subject of multiple chromosome replication (polyteny). In 1947, Hus- 

 kins provided experimental evidence of the existence of such strand 

 multiplication in plant roots by showing that the average number of 

 chromocenters was correlated with the degree of polyteny. Occasionally 

 such cells enter division either spontaneously or by induction but, in- 

 stead of going through a series of "reduction" divisions as in the 

 mosquito gut cells, the nuclei become typically polyploid, with each 

 chromosome having the usual two major strands (Figure 5-8). 



Another kind of "somatic reduction" division was reported by Hus- 

 kins (1947, 1948) following treatment of plant meristems with sodium 

 nucleate. He noted that, at stages from prophase to metaphase, there 

 was a strong tendency for whole chromosomes to separate into two 

 groups. This, if followed by a more or less normal anaphase separation 

 of chromatids, would lead to formation of a multinucleate cell and, with 



MITOSIS AND MEIOSIS / 131 



