CENTRIC CONSTRICTIONS 35 



The centric constriction has the following three characteristics 

 distinguishable at metaphase : — 



(a) The chromosome is more easily bent and stretched at this 

 point. This has been shown by micro-dissection (Chambers and 

 Sands, 1923), and is equally evident from comparative observation 

 of chromosome form in fixed material. The constriction is, as one 

 might expect, a point of weakness in the chromosome. Secondary 

 constrictions share this property. The chromosome, therefore, in 

 the absence of secondary constrictions consists of two limbs separated 

 by the intercalary centric constriction. 



(6) During metaphase the two chromatids lie at this point in the 

 axis of the spindle, although elsewhere lying at random. Evidently 

 the centromere is strictly orientated. The chromosome seen in 

 polar view therefore seems narrowest on either side of the attach- 

 ment constriction. 



(c) The two chromatids are held together tightly at this point, 

 although elsewhere lying loosely parallel. Apparently the centro- 

 mere is either single or its two halves are more closely paired than 

 the rest of the chromatids. The ligature may be short, or may be 

 extended so that a definite non-staining gap occurs in the chromo- 

 some. This also applies to secondary constrictions. 



Constrictions give great diversity to the form of the metaphase 

 chromosomes, and being constant in position and length they also 

 give character to the individual chromosomes. Their study 

 therefore has been of the greatest importance in investigating 

 constancy and variation in mitotic chromosomes within and between 

 species and between parents and their own gametes or gametophytes, 

 particularly in the flowering plants (^.^., in Crepis, Vicia, Tradescantia, 

 Muscari, Tulipa, Drosophila). This advance we owe in the first 

 instance largely to the work of S. G. Navashin and his school, and 

 later to Sakamura (1915 et sqq.), Taylor (1924 et sqq.), Newton (1924, 

 1927), M. Navashin (1926, 1927, 1930), Delaunay (1926, 1929), 

 Lewitsky {loc. cit.). Cf. also Belar (1926), Bridges (1927), Kachidze 

 (1929), Hollingshead (1928). 



The following are the main conclusions which are applicable to 

 the higher plants in general {cf. Delaunay, 1929, and Lewitsky, 



1931). 



