202 THE BEHAVIOUR OF POLYPLOIDS 



somes will be free to pair with their identical homologues, so that 

 sexivalents will occasionally be formed as well as bivalents derived 

 from autosyndesis within the third complement. Such is the 

 condition in " triploid " Pynis Mains (Nebel, 1929 ; D. and Moffett, 

 1930), the diploid form of which is a secondary polyploid (q.v.). 

 Seventeen bivalents are formed in the diploid ; quadrivalents are 

 rare. But in the " triploid " more than seventeen bivalents are 



Fig. 70. — First metaphase in pollen mother-cells of Pyrus [x — ly). 

 Left, diploid forms showing one or two exceptional quadrivalents. 

 Right, triploids : below, 2}, 5", 71", I'v, ivi^ jvn ; above, 51, 

 10", 6'", 2IV. X 2500 (from D. and Moffett, 1930). 



usually formed and frequently associations of six, i.e., the third set 

 pair among themselves (Fig. 70). 



4. In hybrids of hexaploids, octoploids and higher polyploids with 

 diploids. Two, three or more sets of the polyploid are naturally 

 left without a partner, as they are in a haploid and triploid. They 

 are frequently found to pair by autosyndesis. The diploid-hexaploid 

 hybrid not only has the chromosome number of a tetraploid but also 

 behaves like one in pairing. In one such case {Primus) quadrivalents 

 are frequently found in the hybrid, so that pairing is taking place 

 between all three sets of the polyploid parent (Fig. 71). In 

 other cases (Crepis and Papaver) the hybrid can again be crossed 



