MALE GERM CELLS IN NOTONECTA 91 



cut off in the section. The same may be said of figure 106 B 

 where the cross is more condensed. But the probabiHty that 

 these represent stages in the history of the XF-pair is suggested 

 by the occurrence of an unequal tetrad in the late prophase 

 (fig. 106 C, D). The two small components have, in this event, 

 arisen from the longitudinally split short vertical bar of the cross, 

 and the two large components from the split long vertical bar. 

 The small components represent the F-chromosome and the 

 large ones the A'-chromosome. Edward ('11) figures in Ascaris 

 felis the XF-pair in the prophase quite similar to my figure 106 B. 

 As stated previously, the A^- and F-chromosomes are separate 

 in the first division and in figure 106 D from a late prophase they 

 are already somewhat separated. A preliminary separation of the 

 members of the A"F-pair therefore takes place first in the prophase 

 in advance of the other chromosome pairs. This may be corre- 

 lated with the fact stated previously that the X- and F-chromo- 

 somes are frequently found in the second metaphase side by side 

 instead of joined together to form the usual unequal dyad. 



2. Notonecta irrorata. The history of the ring in this species 

 is the same as that described for the M-chromosome of N. insu- 

 lata (fig. 107 A-D). The second longitudinal split remains here 

 also during condensation, and although not seen in lateral view 

 (fig. 107 C) is frequently visible in polar view of the metaphase 

 (fig. 107 D; see also fig. 16). The crosses are likewise similar 

 to those of the other species (fig. 108 A-H). 



3. Notonecta undulata. A detailed study of the prophase fig- 

 ures in this species has not been attempted. They are evi- 

 dently very different from those of N. irrorata and N. insulata 

 and their irregular shape renders them difficult to trace. Some 

 of these in the diffuse stage are shown in figure 109 A~F, and 

 after they have condensed in figure 110 A-F. 



C. Discussion 



The prolonged discussion that has followed Flemming's original 

 discovery of the open ring type of bivalent chromosome is even 

 now not terminated, and the same is true of the cross described 



