THE FUNDAMENTALS OF MENDELISM 



43 



gametophytes might occur before the separation of the chromosomes 

 was complete, and that when the chromosomes appeared again for the 

 second division they still showed the interlocking left over from the 

 first division. 

 A solution of the paradox of the continutity of existence of the 



Fig. 9. Diagram of the Structure of a Chromo- 

 some at anaphase of mitosis. — The centromere is 

 located at the bottom, in the angle of the V, in a 

 non-staining region (the primary constriction). 

 There are two secondary constrictions, one in the 

 right arm and one near the end of the left arm, 

 which cuts off a small lump of chromatin (known as 

 a trabant). In the main body of the chromosome, the 

 chromonema (central thread) is coiled in a spiral; 

 In meiosis this small-scale spiral (minor spiral) may 

 itself be coiled in a larger-scale spiral (major spiral). 

 Some authors maintain that the chromonema is 

 split longitudinally into two threads (see p. 116); 

 it appears to be embedded in a "matrix." The 

 chromomeres, which are best seen in prophases, 

 may persist as swellings on the chromonema. The 

 main part of the chromosome becomes less darkly 

 staining in telophase; there may also be a part, the 

 "inert" or "heterochromatic" part, which remains 

 darkly stained; it is here drawn near the centro- 

 mere. (After Heitz.) 



2 ARY 



CONSTRICTION 



CENTBOMERE 



^ 



%- 2? rw^<^ ^-^/7^ 



\ 





8 - 



Fig. 10. Persistence of Spiral Structure through the Resting Stage. — A late 



telophase, the chromosome thread, which has been tightly wound in a spiral at 

 metaphase, is becoming uncoiled. 6 earliest prophase, the chromosome is coiled 

 to about the same extent as it was at the end of the last telophase. C slightly 

 later prophase, the uncoiling is proceeding, and the thread (which is double) is 

 being thrown into loose "super-spirals" owing to being confined within the 

 nuclear membrane. 



(After Darlington.) 



chromosomes and the homogeneity of the interphase nucleus may per- 

 haps be reached through a study of the spiral structure of the chromo- 

 somes (p. 364). Kuwada and Nakamura^ showed that on treatment with 

 ammonia vapour the spiral thread or chromonema of which the meta- 

 phase chromosome is made becomes unwound and that thereafter the 

 1 Kuwada and Nakamura 1934a. 



