MALE GERM CELLS IN NOTONECTA 89 



figure 98 where the chromosomes are being pulled on to the second 

 spindle immediately after the completion of the first division. On 

 the spindle, the ilf -chromosome lies so that the longtitudinal 

 split is in the plane of the equator; the split therefore marks the 

 line of division (fig. 99). The daughter groups in the late ana- 

 phase of the second division are shown in figure 100. The ilf- 

 chromosome in the second division has rather a peculiar form for 

 this stage. In the metaphase it looks like a tetrad, and after 

 division like a dyad, but this bipartite appearance of the single 

 element has probably no significance. The four chromatids have 

 been distinct since long before the first division and each has 

 retained much the same form throughout its history; this form 

 happens to be a dyad-like structure. 



To sum up: the M-chromosome starts as a double rod which 

 opens out to form a ring; a second longitudinal split appears. 

 In the first division, it divides along the first longitudinal split 

 into what were two half rings. In the second division each part 

 divides along the second longitudinal split which has remained 

 since it was formed. 



It is not only the iVf-chromosome that forms a ring, but the 

 next largest chromosome goes through a similar history, as far as 

 it can be traced. Starting with the open ring which is longitudi- 

 nally split (fig. 101 A), it passes through stages in condensation, 

 exactly parallel with those of M (fig. 101 B~D). In the meta- 

 phase of the first division, in side view, the longitudinal split 

 is not visible since it lies in the plane of the paper (fig. 101 E), 

 but in polar view it divides the chromosome in two halves (fig. 

 101 F). The line of division coincides with the plane between the 

 two half rings. In figure 102 is shown a late anaphase group in 

 which one may distinguish the largest and the next largest chro- 

 mosomes, both longitudinally split. The split in both cases 

 marks the line of separation for the second division. 



In addition to the two large rings, there is a small ring in the 

 prophase, which also has a longitudinal split (fig. 103 A, B). 

 Its history has not been traced. 



b. The cross. At the time that the M-chromosome is leaving 

 the plasmasome, the other chromosomes are on the nuclear wall 



