62 MITOSIS : THE VARIATION OF THE CHROMOSOMES 



has on this account an entirely distinct physiological effect 

 (v. Ch. VII). 



The reciprocal results from the same series of errors as those 

 which give polyploidy and polysomy are found in cells or in whole 

 organisms which lack either individual chromosomes through an 

 error of mitosis {e.g., Zea Mays, McClintock, 1929 c) or meiosis (e.g., 

 Haplo-IV in Drosophila, Plate XII, Fig. 121) or whole chromosome 

 sets (e.g., haploid parthenogenesis, q.v.). 



The table explains the nomenclature of polyploids by examples 

 from a type with a haploid set of five chromosomes (A to E), and 









Fig. 16. — Mitotic metaphases in haploid (after Hollingshead, 1930). 

 diploid, trisomic diploid, triploid and pentaploid (from Navashin, 

 1926) in Crepis capillaris {x — 3, chromosomes labelled A, C and 

 D). X 1500. 



shows the conventional formulae in terms of the haploid number x. 

 Thus X is 5 and a trisomic diploid has 11 chromosomes. 



Since a zygote usually receives two similar sets of chromosomes 

 from its two parental gametes, their number is conventionally 

 referred to as 2n ; where the chromosomes pair regularly at meiosis 

 they therefore form n pairs. Now in a particular individual these 

 2n chromosomes may consist of three sets or four sets of chromosomes 

 relative to its own parents or ancestors. In the present work, 

 therefore, the " basic number " of this ancestral set is distinguished 

 by the sign x. Thus in Triticum vulgar e 2n = 42 and x = y, the 

 somatic chromosome number is therefore hexaploid (6x). Similarly 

 in a trisomic tetraploid form of Primula kewensis (where x = g) 

 2n = 35, and this number will be represented as 4.^ — 1. The 

 endosperm, since it is a characteristically triploid phase of the life 

 of the plant, may be referred to as ^n. 



