NUCLEOLAR CONSTRICTIONS 39 



puted by the Russian school (cf. Sorokin, 1927), who have endowed 

 the trabant with special functions. The observation of intercalary 

 as well as terminal trabants, however, leaves no doubt of their being 

 determined merely by the position of constrictions. 



{d) Where a number of constrictions occur in each chromosome 

 the complement presents an appearance of some complexity. 

 Extreme examples of this are seen in Datura and (Enothera (Lewitsky , 

 1931; D., 1931 d), and in the "tandem trabants" of Allium 

 (Fig. 7 III L). 



(e) The only restriction observable in the free combination of 

 various forms of constriction in different chromosomes is indicated 

 by the rarity of forms with long constrictions separating two large 

 limbs of a chromosome. Further, the long constrictions are 

 commonest where the distal element is smallest, i.e., in a trabant, 

 and they are not usually formed in the shortest chromosomes of a 

 complement with a large size range. Selection has probably played 

 some part in limiting the range of chromosome form, and it would 

 work in two ways. First, a long constriction might not be strong 

 enough to stand the strain of . the anaphase separation with a 

 large element distal to it. Secondly, a long constriction might 

 interfere with the pairing of a short chromosome at meiosis on 

 account of the properties we shall next consider. 



All constrictions are regions of the chromosomes which fail to 

 undergo spiralisation along with the rest. In the case of the 

 centric constriction we can see that this special property is associated 

 with the special functions of the centromere. The question arises 

 as to what special functions are associated with the secondary 

 constrictions. The answer was discovered by Heitz (1931), who 

 found that nucleoli were developed at corresponding positions in 

 daughter nuclei at telophase and, moreover, at positions and in 

 numbers and sizes corresponding to the secondary constrictions of 

 the chromosomes {cf. Geitler, 1935 a). It had been known for some 

 time that nucleoli were associated with trabants at prophase 

 (Navashin, 1926), that trabants were merely distinguished from 

 other constricted bodies by their size (D., 1926), and that the 

 number of nucleoli might vary with the number of chromosomes 

 (De Mol, 1928). These observations were intelligible in view of the 



