68 bulletin: museum of comparative zoology. 



original spireme segment, and its middle point to the distal pole. The 

 rod-shaped tetrad becomes oriented in the spindle of the first matura- 

 tion division with its long axis parallel to the spindle-axis, and at 

 metaphase separates in the middle. In other words, the plane of the 

 secondary split becomes the plane of the first maturation division, 

 which is therefore equational. If now we may assume that the longi- 

 tudinally split spireme segment has represented a pair of chromatin- 

 threads which had conjugated side-by-side throughout their length, 

 the plane of the primary split must be the plane of the reductional 

 division, which becomes effective in the second spermatocyte mitosis. 



The tetrad A also forms rings, as shown in figure 62, j, k, I (Plate 6). 

 I have not been able to trace these rings into the metaphase to de- 

 termine their orientation on the spindle, and furthermore I am quite 

 uncertain whether the ring shape persists as far as the metaphase. 

 Most of the metaphase figures show one tetrad in the form of a rod 

 with its axis parallel to the spindle-axis, and with a constriction in the 

 middle, as shown in figure 62, i and figure 79, A (Plate 7). Sometimes 

 two or more rod-shaped tetrads are to be seen in the same spindle 

 and with the same orientation. However, one of them is alwavs in a 

 more advanced stage of division than the others, and I have been in- 

 clined to identify this precocious one with tetrad A. Figure 62, c-t, 

 indicates that such a conclusion is justified. Since the straight-rod 

 condition is so characteristic of the metaphase, it may be that the 

 rings also become transformed into straight rods by the time the 

 metaphase is reached. 



The rings seem to have been formed either by a failure of the proxi- 

 mal ends to separate during the formation of the secondary longi- 

 tudinal split, or by a secondary union of these ends, i. e. after the split 

 had begun. For example, if a tetrad in the condition of figure 62, c, 

 has the secondary split completed without the separation of the proxi- 

 mal ends, a ring would result. So also would a ring be formed by a 

 secondary union of the two proximal ends of a stage such as is seen in 

 figure 62, d or e. In either event the region within the ring would 

 represent the space formed as a result of the secondary longitudinal 

 split. If the chromatids should now begin to separate at the proxi- 

 mal end along the plane of the primary split, as seems to be indicated 

 in figure 62, k and /, and if this process should be continued until a 

 metaphase chromosome such as that shown in i is produced, there is 

 every reason to believe that it would result in a separation of the 

 original conjugants of the pair, and therefore constitute a reductional 

 division. On the other hand, it is possible that the separation along 



