Figure 4-15. Photomicrograph of First Meiotic Metaphase in Micro- 

 sporogenesis of Trillium erectiiin Showing Two Supernumerary or Fragment- 

 Hke Chromosomes. (From Sparrow, A. H.. Pond, V.. and Sparrow, R. C, 

 1952. "Distribution and Behavior of Supernumerary Chromosomes during 

 Microsporogenesis in a Population of Trillium erectum L.," Am. Naturalist, 

 86, Fig. 4, p. 282. Courtesy of Dr. A. H. Sparrow, Brookhaven National 

 Laboratory. ) 



Chromomeres 



At certain stages, most notably prophase of the first meiotic division, 

 the chromosome often may be seen to be differentiated longitudinally 

 by chromatic thickenings known as chromomeres (see Chapter 5, Figure 

 5-7). The question of their functional significance is somewhat debat- 

 able, although the fact that the number, and to some extent, even the 

 size is constant suggests the tempting hypothesis that they represent the 

 genetically significant part of the chromosome. No conclusive proof of 

 this idea has ever been provided and it should be noted that hetero- 

 chromatic regions are not without chromomere-like structures. The exact 

 structure of the chromomeres has not yet been worked out to the point 

 where there is any general agreement. Belling considered them to be 

 chromatic thickenings surrounding a small granule which he referred to 



94 / CHAPTER 4 



