HETEROPLOIDS 



185 



the primaries, but the frequencies are different. The 

 most noticeable features are seen in the last two types (to 

 the right). One of these is an elongated ring of three 

 chromosomes, the other is a ring of two chromosomes and 

 a small single-ring chromosome. These two types suggest 



A Z 



Z A 



AZ 



X 



AZ 



Fig. 105. 



Diagram illustrating possible conjugation of two chromosomes, 

 turned in opposite directions. 



that, in some way, the end of one chromosome has been 

 changed. Belling and Blakeslee offer the following provi- 

 sional suggestions as to how such a change may have been 

 brought about at a preceding stage in the triploid parent 

 or in a trivalent of a primary type. Suppose, for example, 

 two chromosomes should come to lie side by side in re- 

 versed position as shown in Fig. 105, and suppose they 

 should cross over in the middle, which is the only level 



