90 ETHEL NICHOLSON BROWNE 



in the form of thin double rods (fig. 60) . The genesis of the cross 

 from these bodies is as follows. The two segments open out into 

 a V, each arm of which becomes longitudinally split (fig. 104 A). 

 The arms of the V open out still further so as to form a double 

 straight rod, the original space between the arms (i.e., the first 

 longitudinal split) being represented only by the small opening 

 in the middle of the two bars (fig. r04 B). There are e\Hdently 

 two methods by which a tetrad may be formed from this figure. 

 The double bars may condense, while the connection around the 

 central opening becomes very thin (fig. 104 C, D), or the connec- 

 tion around the central opening may become pulled out trans- 

 versely, so as to form the cross-bars of a typical cross (fig. 104 

 E, F) . In this case, half of each long arm and half of each short 

 arm condense to form one element of the tetrad (fig. 104 G). 

 The end result is the same in either case, a tetrad is formed in 

 which the original longitudinal split is represented by the division 

 line through the short axis and the second longitudinal split by 

 the line through the long axis. In the metaphase, the tetrad 

 lies with its long axis parallel with the spindle and its short axis 

 in the plane of the equator. The first division therefore separates 

 the two • components of the original double rod. The vertical 

 split is usually rather difficult to make out with certainty in the 

 metaphase but in some cases is quite clear (fig. 104 H). This 

 split becomes very distinct in the anaphase, and marks the line of 

 separation of the second division (fig. 104 /). 



c. The double rod. By a process of condensation, the original 

 double filament forms a thick double rod, the two components 

 of which lie parallel (fig. 105 A, B). These become united at 

 one end, and straighten out to form a dj^ad (fig. 105 C, D). There 

 is no clear evidence of the presence of a second longitudinal split. 

 In the metaphase, the chromosome lies with its original longitudi- 

 nal split in the pla^ne of the equator, so that the first division 

 separates the two components of the original double rod. 



d. XF-pair. In figure 106 A is shown a difi'use cross which 

 differs from the ordinary cross described above only in the fact 

 that its longitudinal bars are unequal. It is possible, of course, 

 that this is an ordinary cross of which part of one bar has been 



