MALE GERM CELLS IN NOTONECTA . 87 



3. Noto7iecta undulata. In this species also, the first indication 

 of the final chromosomes is the presence on the nuclear wall of 

 thin, more or less coiled threads (fig. 85). The prophase figures 

 which these form are quite different, however, from those of the 

 other two species. At first these have a very vague, spongy 

 appearance and are coarsel}^ granular (fig. 86). By a process of 

 condensation, they become more compact and more definite in 

 outline (fig. 87). The figures are quite irregular in shape, but 

 in general consist o*f two bars, diverging or united at one or both 

 ends. While in this stage, the nuclear membrane breaks down 

 and the spindle fibers begin to form. The chromosomes are still 

 quite irregular in shape after the spindle is fully formed (fig. 88) , 

 and do not assume their definitive form until the full metaphase. 



B. Detailed description 



1. Notonecta insulata. a. The ring. The M-chromosome is 

 usually the last one out of the plasmasome, and is therefore in the 

 interior of the nucleus at the time that all the other chromosomes 

 are in a diffuse condition on the nuclear wall (fig. 60). Owing 

 to this fact and also to its greater size, its history can be traced 

 throughout the prophase and also during the first and second 

 maturation divisions. Whereas the other chromosomes come out 

 of the plasmasome in more or less irregular masses, the M-chromo- 

 some has the form of two rods, somewhat coiled about each other, 

 but in general taking the same direction (fig. 89 A-D). The two 

 rods untwist, and open out in the middle, usually becoming or 

 remaining united at the ends (fig. 90 A-D). By opening out 

 still more, a small ring is formed (fig. 91 A-U) ; frequently at this 

 stage and occasionally earlier, a longitudinal split is present, in 

 one or both half rings. If we term the original line of separation 

 between the two rods a longitudinal split, this is the second longi- 

 tudinal split. B}^ this time, the M-chromosome has reached the 

 nuclear wall, and at once a process of expansion sets in. The, 

 ring opens out until the enclosed space becomes relatively very 

 large and the ring itself correspondingly thin (fig. 92 A, B). It 



