4 ANNALS NEW YORK ACADEMY OF SCIENCE;; 



cells the chromosomes are long rod-shaped structures very uniform in 

 size, and each is clearly a single chromosome and not a group of two 

 or more chromosomes. Jn these figures the chromosomes are often rather 

 closely aggregated and more or less intertwined. But whenever the 

 chromosomes are spread out in an early stage of the equatorial plate 

 sufficiently to admit of accurate count, the number has been found to be 

 most frequently 12 as shown in PI. I, Fig. 1. But the chromosomes may, 

 in some cases at least, divide and separate quickly and. very completely, so 

 that one can best determine the correct somatic number during the early 

 stages of the equatorial plate (PI. I, Fig. 1), or later in the anaphases 

 when all the chromosomes have completely divided and the two sets are 

 fully separated (PL I, Fig. 2). 



Irregularities in the Number of Chromosomes in Somatic Cells 



For many cells in the embryonic regions of the root tips and of the 

 stems the distribution of the daughter chromosomes during the anaphases 

 is regular, resulting in the passing of 12 chromosomes to each daughter 

 nucleus (PI. I, Fig. 2). However, cases of unequal distribution involving 

 non-distribution* of certain pairs of daughter chromosomes are to be 

 found which result in daughter nuclei with the numbers 11 and 13, or of 

 10 and 14. Still greater inequalities of distribution are to be seen. Espe- 

 cially when cells are of asymmetrical shapes, they may divide in such a 

 way that only a few chromosomes pass to one pole. This La shown in PI. I, 

 Fig. 3, in which there is a normal total of 24 chromosomes in an anaphase 

 stage, but non-distribution is so general that 19 chromosomes are collect- 

 ing at one pole. Thus in a single mitosis of a somatic cell that had L2 

 chromosomes, the number of chromosomes which a daughter nucleus may 

 receive may rise to at least as many as 19. 



In the division of nuclei which have previously received an increased 

 number of chromosomes, further non-distribution of chromosomes may 

 occur, giving nuclei with varying numbers of chromosomes above the 

 normal of 12, and in some cases the number may rise to more than 

 double the normal diploid number. 



* The term "non-distribution" will be used In this paper to designate those cases 

 either In Bomatic or In meiotlc divisions where two daughter chromosomes form and 

 separate by the splitting oi a mother chromosome, bul arc not distributed to different 

 daughter nuclei. The term "non-disjunction" applied by Bridges (1013) will be here 



employed only fur cases where two homologous chromosomes of a pair arc not dis- 



tributed to dlfferenl daughter nuclei during the meiotlc divisions. According to 

 Wilson (1925, p. 872) and to Sharp (1926, p. i* m • » the term nondisjunction can be 

 applied i>"ih to the aon-dlsjunction of homologous chromosomes during meiosls and 

 to the Qon-dlstributlon of sister chromosomes in somatic divisions, bul a distinction as 

 here suggested Beems highly desirable 



