5o6 



CELL MECHANICS 



whether there was any possibility of exchanges in linear connection 

 amongst the threads such as might determine genetic crossing-over 



FiNAI 







VJf 



Dip/of-ene. 



DiaJ^ 



UaKinests. 



rief'apne, 



Fig. 141.— Diagram showing different types of terminalisation. 



a, b, c : Fritillaria type with slight change of position and 

 little or no reduction in number of chiasmata through fusion. 

 d, e, f : Campanula type in which the movement is completed 

 and results in terminalisation of all chiasmata. Tulipa, Lilium 

 and Rosa are intermediate between the two. b shows that the 

 same amount of movement as gives little change of appearance in 

 the large chromosomes, in a fragment (of Fritillaria irnperialis) 

 gives complete terminalisation. c^ a characteristic bivalent in F. 

 Meleagris with localised chiasmata, shows that movement 

 occurs in a distal chiasma although scarcely evident in the proxi- 

 mal one. e, f, show alternative behaviour to d where chiasmata 

 are formed only on one side of the centromere, but con- 

 ditions are otherwise the same. This explains the origin of the 

 two types of bivalent form in " type C " in Phrynotettix (Wenrich, 

 1916, cf. Fig. 86), and " type 7 " in Circotettix (Helwig, 1929). 



(Ch. VII). With the same end in view a record of the development 

 of chiasmata was necessary in order that their number and position 

 at the moment of origin (before they can be recorded) might be 

 inferred. Finally, a comparison of analysis of hybrids with that of 



