pinney: organization of the chromosomes 313 



IN PHRYNOTETTIX MAGNUS. 



Schreiner, in their studies of Spinax niger and Myxine gluti- 

 nosa, have described a looped arrangement of the thread dur- 

 ing the conjugation period similar to figures 16 and 17. Here 

 conjugation takes place by the lengthwise union of entire 

 threads, each chromomere of one uniting with a homologous 

 chromatin unit of the other, thus forming a thicker chromatin 

 thread with a longitudinal division. No such union of threads 

 as they describe has been observed in my material. The divi- 

 sion in the thread precedes that in the polar granule, and often 

 we find several polar granules united with the radiating 

 threads showing a longitudinal split (figs. 19, 20 a). Here 

 again we have a resemblance to one of the figures of the above 

 authors, in which they find the chromatin threads radiating 

 from a chromatic body which they call the "Knotenkorper." 

 Figure 19 shows the accessory chromosome with no threads 

 attached. 



By a subsequent contraction and longitudinal splitting of 

 these threads in figure 16, tetrads like those shown in figure 

 18 are formed. The interesting and peculiar feature of these 

 formations are the condensed bulbous thickenings marking the 

 synaptic end of each chromatid. These changes have taken 

 place within the nuclear membrane. The accessory has as- 

 sumed a regular form and still maintains its lateral position. 

 The lesser bodies, grouped as represented in figures 16 and 17, 

 are missing unless we identify them with the condensed thick- 

 enings on the ends of the chromatids. As the chromatids 

 coalesce the longitudinal divisions of the thread and its homo- 

 geneous ends are obliterated, and we find many figures re- 

 sembling figure 18/. Oftentimes these peculiar enlargements 

 occur at each end of a chromatid. This is confusing and, in 

 the light of my limited observations, unaccountable. Figures 

 21 to 27 show the usual succession of changes in the first 

 spermatocyte division. There are twelve chromosomes in 

 the metaphase. Figures 22 and 23 show the constancy of 

 form which prevails within the species. There are always two 

 large rings, six smaller rings, two very short rods, and one 

 longer rod. The accessory, homogeneous throughout the pro- 

 phases, becomes rough in outline during the brief moment of 

 its journey to the pole, but regains its smooth contour in the 

 telophase of the first spermatocyte (fig. 28). A slightly later 

 telophase shows the accessory less condensed but apparently 



