THE MORPHOLOGY OF THE CHROMOSOMES 



115 



Noteworthy in this connection is the fact that the characteristic 

 size differences shown by the several chromosomes of the same nucleus 

 are almost wholly a matter of length; commonly the diameter of all of 

 them is the same. This suggests that chromosomes are composed of 

 smaller units arranged in a row, and that the longer chromosomes of a 

 group have more units than the shorter ones. This interpretation may 

 not be universally applicable, but genetical evidence to be presented 

 in later chapters indicates its general validity. 



Spindle -attachment Region. — The metaphase or anaphase chromo- 

 some usually shows at some point a differentiation which has commonly 



Fig. 60. Fig. 61. 



Fig. 60. — A chromosome with submedian spindle-attachment region passing to the pole 

 in anaphase. The shorter arm has a "secondary constriction." 



Fig. 61. — Diploid chromosome complement at metaphase in root tip of Vicia faba. 

 Spindle-attachment regions of all chromosomes near equator. Nucleolus-forming regions 

 are by the "secondary constrictions" above and below. In this strain one pair of short 

 chromosomes has conspicuous "constrictions" also (left and right). X1750. 



been referred to as a "constriction," since it appears as a relatively 

 slender region in most preparations showing it clearly in these stages 

 (Fig. 60). Careful studies have shown that the region in question 

 differs from the rest of the chromosome in its structure and achromatic 

 nature, and not merely in apparent diameter. In some preparations, 

 especially of the prophase, it may appear as wide as or even wider than the 

 adjacent portions; hence the term "constriction" is not truly descriptive. 

 Some chromosomes have two or more achromatic regions. It is a 

 striking fact that in a given chromosome these have definite and con- 

 stant locations, 2 although they may be very differently placed in different 

 chromosomes. 



2 Exception should of course be made for individuals or races in which their posi- 

 tions differ because of translocation, inversion, or other alteration. See McClung 

 (1917), Carothers (1921), King (1923), Helwig (1929); also McClintock (1931) on 

 Zea. It was found by Sakamura (1920) that chloral hydrate shortens and thickens 

 the chromosomes, thus accentuating the "constrictions." The effects of fixation 

 have been studied by S. and M. Nawaschin, Lewitsky (see 19316), and others. 



