162 INTRODUCTION TO CYTOLOGY 



members of the group. It has been shown in certain instances that these 

 constrictions have constant positions in the chromosome. A careful 

 study of this phenomenon has been made by Sakamura (1915, 1920). 

 In Viciafaba, for example, he finds that each of the two long chromosomes 

 ("M-chromosomes") of the somatic group has two constant constrictions, 

 one at the middle and one near the end ("m-constriction" and u e-con- 

 striction") (Fig. 56, A). The m-constriction marks the point of attach- 

 ment of the spindle fibers. There are also end-constrictions in 8 of the 10 

 short chromosomes. On the basis of the widespread occurrence of con- 

 strictions in the chromosomes of both plants and animals Sakamura has 

 interpreted a number of puzzling phenomena, such as the apparent vari- 

 ation in chromosome number within the species (see below) and certain 

 features of the reduction process (Chapter XI) 



Such regularly situated constrictions have also been demonstrated in 

 Fritillaria tenella by S. Nawaschin (1914). Here they are present at the 

 middle of the largest chromosomes, nearer one end in the medium-sized 

 chromosomes, and close to the end of the smallest ones. In Crepis virens 

 (M. Nawaschin 1915) there are constrictions near one end in two of the 

 1 hree chromosomes of the haploid group in the pollen grain, in four of the 

 six chromosomes of the diploid group in the somatic cells, and in six of 

 the nine chromosomes of the triploid group in the endosperm cells. Such 

 a definiteness in the location of constrictions was also seen earlier by Agar 

 (1912) in the chromosomes of the fish, Lepidosiren. 



Somewhat similar evidence has been brought forward by Wenrich 

 (1916), who finds that the chromatic lumps, or chromomeres, have a 

 striking constancy in position as well as in size in the chromosomes of 

 Phrynotettiz (Fig. 155). Wenrich (1917) also reports that the small 

 "chromomere vesicles" attached to the chromosomes of certain orthop- 

 terans always appear at definite points along the chromosome (Fig. 55, C). 

 It therefore appears that the chromosomes of a given group or comple- 

 ment not only maintain a genetic continuity from cell to cell, but are also 

 in some way qualitatively different from one another. They are conse- 

 quently said to have a specificity as well as an individuality, or continuity. 

 The relatively constant positions of the constrictions, chromatic lumps, 

 and chromomere vesicles afford further visible evidences that the chrom- 

 osome may possess some kind of lengthwise differentiation, a fact which, 

 if clearly demonstrated, would be of the highest importance in connection 

 with current views of the role of the chromosomes in heredity. (See 

 Chapter XVII.) The significance of chromosome constrictions in this 

 respect has been emphasized by Janssens (1909), S. Nawaschin (1915), 

 and Sakamura (1920). 



Chromosome Number. 1 — It was long ago noticed by Boveri, van 



1 For lists of chromosome numbers in plants see Ishikawa (1916) and Tisehler 

 (1916). For the numbers in animals see Harvey (1916, 1920). 



