92 



FUNDAMENTALS OF CYTOLOGY 



Meiotic chromosomes are especially favorable for the study of 

 chromonemata, first of all because in higher plants they occur in cells 

 (microsporocytes) that lie more or less free from each other in the anther. 

 They can therefore be easily pressed out on a slide in large numbers 

 for study in the living condition or for special treatments. In the pro- 

 phase of the first of the two nuclear divisions in the microsporocyte 

 they are usually longer and straighter than at any other period in the 

 life cycle; moreover, thej^ contain very little stainable material except 

 in the chromomeres. Hence in some plants, e.g., maize, the more minute 

 structural features of different chromosomes can be clearly seen and 



\ 





Fig. 64. — Chromosomes (synapsed pairs) of maize at mid-prophase in microsporocytes. 

 a, chromosome 6 attached to nucleolus by its nucleolus organizer (compare Fig. 58). b, 

 portion of chromosome 8, showing chromomeres. c, chromosome 7 with heterochromatic 

 region next to kinetochore at right, knob at left, d, B-type chromosome seen in certain 

 strains; euchromatic region above, and heterochromatic region below, e, portion of 

 chromosome 9 in a strain heterozygous for knob size. /, chromosome 9 with terminal 

 knob. (After B. McClintock.) 



closely compared (Figs. 58, 79). One result of such studies on plant 

 microsporocytes and animal spermatocytes has been to show that the 

 various chromomeres tend to appear in regular and constant patterns 

 in particular chromosomes. In maize and its relatives large chromatic 

 knobs also occupy definite positions. This characteristic longitudinal 

 differentiation of the chromosome suggests a corresponding functional 

 differentiation, and this conception is borne out b}^ the results of cyto- 

 genetical studies. 



In the meiotic prophase other structural features, such as kinetochores, 

 heterochromatic regions, and nucleolus organizers, also stand out clearly 

 (Fig. 64). The nucleolus organizer in maize appears as a swollen hetero- 

 chromatic region immediately proximal to the secondary constriction in 

 the shorter arm of chromosome 6. In the meiotic prophase the nucleolus 

 formed at the preceding telophase is still present in contact with it. 

 That the heterochromatic region rather than the constriction itself acts 

 as the organizer is shown by the fact that in abnormal cells which have 



