No. 463.] STUDIES OF PLANT CELL.— VL 485 



dently out of the reticulum. The two thi'eads gradually fuse so 

 that in older stages of synapsis the nucleus appears to contain a 

 single relatively thick spirem which is shorter and more loosely 

 coiled than in the earlier stages. The minute structure of the 

 threads of the spirem can be determined by careful staining. 

 They consist of a series of chromatin granules (chromomeres) 

 imbedded in the ground substance, linin. As the two threads 

 fuse the chromomeres generally come together in pairs and unite 

 to form a single row of large chromomeres which project from 

 the side of the larger single (fusion) spirem. 



The single (fusion) spirem on emerging from synapsis becomes 

 uniformly distributed throughout the nucleus. There appear to 

 be no free ends in the much convoluted and looped thread. 

 Some of the loops become fastened to the periphery of the 

 nucleus but there is no regularity in the number of loops and no 

 relation to the number of chromatic segments that are formed 

 later. While thus evenly distributed the single spirem under- 

 goes a longitudinal fission which is preceded by the division of 

 each chromomere. This is the first longitudinal fission of the 

 sf)irem which is well known through the descriptions of Guignard, 

 Gregoire, Strasburger, Mottier, and others. The fission is not 

 simultaneous throughout the length of the spirem, for some por- 

 tions remain undivided for some time when contiguous parts are 

 plainly split. The result is a condition very similar to that pre- 

 sented just before the fusion of the two systems of threads 

 during synapsis which produced the single (fusion) spirem. It 

 seems probable that the threads which become separated are 

 morphologically the same as those which fused during synapsis 

 although the union at that period seems complete. The split 

 spirem remains uniformly distributed throughout the nucleus 

 exhibiting, however, a tendency to become somewhat massed in 

 the center of the nuclear cavity leaving fewer loops attached to 

 the nuclear membrane. 



The split spirem now segments throughout its length into the 

 reduced number of chromosomes (12) characteristic of the 

 heterotypic mitosis. The segmentation is not simultaneous, but 

 the first free ends appear near or at the periphery of the nucleus 

 where the split spirem breaks apart at the loops. As segmen- 



