half chromosomes do in mitosis, and Darlington has extended this notion 

 to explain chiasma formation as the result of strain in the twisted homo- 

 logues. Although the theory is logical in its main outline, it fails to 

 explain a number of known facts: first, homologous chromosomes have 



f 





Figure 7-2. Photomicrograph Showing Helical Coil in Meiotic Chromo- 

 some of Trillium. (Courtesy of Dr. A. H. Sparrow. Brookhaven National 

 Laboratory.) 



a random direction of coiling with respect to each other; second, the 

 relational coil is not developed at prophase but simultaneously whh the 

 development of the helical coils; and third, the direction of coiling is not 

 uniform throughout the chromosome, being random across the kineto- 



166 / CHAPTER 7 



