104 



Cellular Structure and Activity 



to form chromosomal fibers and do not move 

 on the spindle (akinetic fragments, Carlson, 

 '38). In the case of the diffuse kinetochore 

 any piece of the chromosome becomes at- 

 tached to the spindle and moves normally 

 (Hughes-Schrader and Ris, '41). The diffuse 

 kinetochore is found in certain insects — the 



B 



C 



%. 



Fig. 19. Structure of the kinetochore. A, Meiotic 

 chromosomes of Trillium (after Matsuura, '41); 

 ch. = chromonemata, K.M. = kinetochore matrix, 

 a = metaphase, b-d = early anaphase. B, Meiotic 

 chromosome of Amphiuma (after Schrader, '39); 

 ch.f. = chromosomal fiber, K.S. = kinetochore 

 spherule. C, Pach5rtene chromosome of rye (after 

 Lima-de-Faria, '49); K. = kinetochore, c.chr. =z 

 centromeric chromomeres. 



Hemiptera, Homoptera and probably also 

 Odonata (Oksala, '43) and Lepidoptera (Su- 

 omalainen, '53) in a myriapod (Ogawa, '49), 

 a few scorpions (Rhoades and Kerr, '49), 

 and in a group of plants (Malheiros, de Cas- 

 tro and Camara, '47). The kinetochore in 

 Ascaris, often described as multiple, probably 

 also falls into this category. 



New insight into the nature of kineto- 

 chores may come from a further study of 

 the accessory kinetochores (neocentric re- 

 gions) that turned up in some strains of 

 rye and maize (reviewed by Rhoades, '52). 

 During meiotic divisions secondary chromo- 

 somal fibers develop in some of the chromo- 

 somes in addition to those formed by the 



regular kinetochores and prematurely pull 

 the chromosome ends to the poles. Especially 

 interesting is the observation that in maize 

 these neocentric regions form chromosomal 

 fibers only if they are in physical connec- 

 tion with the primary kinetochore (Rhoades, 

 '52). 



The microscopic structure of the kineto- 

 chore is still imperfectly understood. The 

 clearest photographs are those of the kineto- 

 chore in Trillium, published by Matsuura 

 ('41). It appears to be a section of the 

 chromonema that remains ixncoiled. On the 

 spindle it is surrounded by a hyaline material 

 (kinetochore matrix) that divides in early 

 anaphase (Fig. 19^). In chromosomes of 

 rye, onion and other plants the kinetochore 

 was shown to be an uncoiled region of the 

 chromonema with a pair of "centromeric 

 chromomeres." It has been suggested that 

 the doubleness is due to an inverse repeat 

 (Fig. 19C) (see Lima-de-Faria, '49, '50). 



It is probable that such special "chromo- 

 meres" or heterochromatic knobs of the ki- 

 netochore region in the chromonema are 

 identical with the "spindle spherule" which 

 has been described in chromosomes of sev- 

 eral plants and animals (Fig. 19S, of. 

 Schrader, '39). Normally the kinetochore 

 divides lengthwise like the rest of the chrom- 

 onema. A number of cases are known, how- 

 ever, where it divides transversely (mis- 

 division), giving rise to isochromosomes (for 

 instance, Miintzing, '44). The kinetochore 

 may occasionally be broken into two func- 

 tional fragments (McClintock, '32). Such 

 terminal kinetochores, however, are usually 

 not stable (Rhoades, '40). 



CHROMOSOMAL FIBERS. Of all the mitotic or- 

 ganelles, chromosomal fibers are most di- 

 rectly involved in chromosome movement. 

 They develop between the kinetochore and 

 the centrosome or spindle pole. In some in- 

 stances they may form in the absence of an 

 organized spindle (Peters, '46; Rhoades, '52; 

 Scott, '36; Pease, '41). Under favorable con- 

 ditions they are visible in the phase micro- 

 scope in living cells (whitefish. Fig. 12C) 

 (Fell and Hughes, '49; Tahmisian, '51). 

 After fixation they appear as a bundle or a 

 sheet of fibers thicker near the kinetochore 

 and tapering toward the poles. They stand 

 out distinctly in the polarizing microscope, 

 also in the living cell (Inoue, '52), owing to 

 their strong birefringence that is positive 

 with respect to their long axis (Fig. 17^4). 

 Like the spindle body they have been re- 

 garded as either positive or negative tactoids 

 (Bernal, '40; Ostergren, '49). Just as in the 



