THE MORPHOLOGY OF THE CHROMOSOMES 125 



chromosomes respectively, as in the foregoing "diploid" types, but other 

 numbers, such as two and four sets (" tetraploid " types), three and six 

 sets ("hexaploid" types), and so on. Such individuals or species are 

 said to be polyploid. This phenomenon and its significance are to receive 

 detailed consideration in Chapters XX to XXII. 



The Arrangement of the Chromosomes in the Mitotic Figure. — 

 As a general rule it seems that the various chromosomes of the comple- 

 ment have no constant relative position in the nucleus or mitotic figure. 

 A comparsion of their positions in a number of metaphases in the same 

 plant usually shows that the arrangement varies greatly. It frequently 

 happens, however, that the arrangement is the same in figures which lie 

 near each other, as in the same row of cells in a root tip, or in neighboring 

 sporocytes. This is easily understood when it is remembered that the 

 halves of the chromosomes pass rather directly to opposite poles from 

 the equatorial plane and thus form similar patterns in the sister nuclei 

 in the telophase. If the next mitosis follows soon, this pattern may be 

 retained through another nuclear and cell generation. As the division 

 rate becomes retarded and the cells compared come to lie farther apart, 

 the arrangement is somehow altered. To what extent the chromosomes 

 may change their relative position in interphasic and metabolic nuclei is 

 not known, but irregularities in their grouping at the end of the anaphase 

 seem to be in part responsible for the alteration observed. 



What has been said above applies particularly to the elongated somatic 

 chromosomes of plants whose morphology has been most studied. An 

 examination of the chromosomes in certain sporocytes, where they are 

 short and well separated from one another in the developing mitotic 

 figure, shows that they tend strongly to assume arrangements like those 

 of floating magnets of the same relative dimensions; they behave as if 

 they were all acting under a mutual repulsion, especially of their spindle- 

 attachment regions. The most stable arrangement occurs most 

 frequently, other less stable ones being due to such modifying factors 

 as the viscosity of the medium and the time required to assume the 

 arrangement.^^ 



In animals also, particularly in the insects, cases are known in which 

 the chromosomes show a more or less constant and characteristic arrange- 

 ment in the metaphase. Often the smaller chromosomes lie in the middle 

 of the figure with the larger ones around them (Fig. 215), or they may 

 form a circle with certain members inside or outside it.^ Here again 

 the phenomenon is best displayed where the size and spacing of the 

 chromosomes are such as to permit their free movement as the metaphase 

 stage develops. In the Diptera, as already mentioned, the chromosomes 



i^vuwada and collaborators (1929). Cf. R. S. Lillie (1905), Cannon (1923), 

 Midler (1912), and Heimans (1928). 



2" See Wilson (1925, various figures in Chap. XI; also 1932). 





