24 MARGARET C. FERGUSON 



their free ends (fig. 12, #, c, d, e). Even in such a late stage of 

 fission as that represented in fig. 13 the sister threads can almost 

 invariably be traced, but not always, as some are out of focus 

 and others are doubtless in another section. 



The stages immediately following longitudinal splitting and 

 segmentation of the nuclear spireme are somewhat different 

 from any that I have seen described by other writers. So 

 puzzling were they to me when the study of microsporogenesis 

 was first undertaken in 1899 that a paper, partially prepared at 

 that time, was laid aside until a larger experience with cell 

 structures could be brought to bear upon this, which is to me 

 at once one of the most intricate and interesting problems con- 

 nected with the activities of the cell. As stated in the intro- 

 duction, new material was collected in 1901 and fixed with great 

 care. Many slides were subsequently prepared, and the phases 

 in the tetrad-division were found to accord perfectly with those 

 observed during the first period of study. The interpretation 

 of the phenomena noted is, however, much more satisfactory 

 now than formerly, although there is still much that is obscure. 

 Sporogenesis has not been studied in Pinus montana var. un- 

 cinata^ but there is complete accord, except in such details as 

 have already been mentioned, in the other four species. 



Longitudinal division is scarcely more than completed when 

 the double skein begins to contract, the two halves of each seg- 

 ment twisting upon each other to a greater or less degree and 

 gradually fusing. As the segments contract the sister-halves 

 may frequently become more or less twisted upon each other ; 

 they may appear as parallel threads ; the half segments may 

 separate at both ends, remaining united at the middle only ; 

 or, having fused at both extremities, they may open out, 

 forming rings (figs. 12 and 12, a). Fusion invariably begins 

 first about those nucleoli which have still retained, although 

 in a less degree than prior to synapsis, the power to react to 

 chromatin-stains (fig. 13). Contraction and fusion continue 

 until a coarse, more or less anastomosing structure is formed 

 in which only traces of the earlier longitudinal division re- 

 main evident (fig. 14, plate II), and a little later all signs of 

 fission, both longitudinal and transverse, disappear (fig. 15). 



