520 University of California Publications in Zoology [VOL. 11 



5. More posterior in the testes the Y-figures are replaced by 

 loops showing the same polarization, i.e., with the free ends 

 directed toward the sphere while the bends are found in the 

 region opposite to the centrosomes. 



6. At this stage the fine threads can no longer be detected and 

 the loops appear as single threads. 



This evidence leads to the conclusion that the loops are formed 

 by the union of the fine threads. A more critical examination 

 of the early stages raises the question as to what are the relations 

 of the conjugating leptotene threads to each other. Up to this 

 point the individual V- or Y-figures described in any one nucleus 

 have been considered as unrelated to each other. If there are 

 only fourteen bifid figures in any one nucleus, each must be 

 considered one end of a loop in the later polarized stage, the 

 other end of the loop not yet having come into existence. But 

 since there are about twenty-eight pairs of threads at the begin- 

 ning of polarization and since the twenty-eight free ends of the 

 completed loops- directed toward the proximal pole correspond 

 in position to the stems of the Ys, it leads one to think that the 

 two ends of each loop are formed simultaneously and before the 

 middle part comes into existence. 



While the foregoing is strong evidence that the stem of each 

 Y-figure becomes one end of a polarized loop, the way in which 

 the two ends of each loop become associated remains to be con- 

 sidered. It will be remembered that each one of the fifty-six 

 short, fine, leptotene threads which unite in one way or another 

 to form the fourteen loops is indirectly continuous with the others 

 through the medium of the network. The further evolution of 

 these threads might be thought of in one of two ways. In the 

 first place the fifty-six threads might be imagined as separate, 

 individual filaments which pair to form the twenty-eight Y- 

 figures; in the second place, they may be conceived of as not so 

 many independent parts, but as so definitely related that each 

 branch of each Y would be one end of a potential thread, the 

 other end of which would be represented by one of the branches 

 of some other Y. 



In regard to the first of these possibilities, it might be argued 

 that the arms of the Y-figures on one side meet and join with 



