LIFE HISTORY OF PINUS 25 



As the thread thickens and broadens it becomes irregular in 

 outline, the irregularities increase, those from neighboring por- 

 tions of the threads meeting and fusing. Soon afterwards a 

 transverse division again becomes apparent (fig. 16). The 

 segments continuing to shorten and thicken gradually draw 

 away from one another, finally remaining united only by 

 delicate threads ; the connecting fibers are at last severed and 

 the chromosomes lie free in the nuclear cavity. The usual 

 number of segments formed is twelve, although thirteen, four- 

 teen, and, in rare instances, as many as sixteen have been 

 counted (figs. 16, 17, 18, a-c, and 20). 



The chromosomes thus arise from an incompletely reticu- 

 lated structure rather than directly from the spireme. While 

 this suggests the condition in magnolia where, as recently 

 described by Andrews ('01), the chromosomes arise directly 

 from the resting reticulum without the intervention of a spireme, 

 it is, in matter of fact, very different. We have here not a 

 nuclear reticulum in the ordinary acceptation of that term, but 

 a somewhat reticulated structure formed by the anastomosing 

 with each other, at certain points of contact, of adjacent por- 

 tions of a previously longitudinally split spireme. As the 

 chromosomes separate out almost every conceivable form may 

 be found, not only the X's, Y's and V's of Belajeff, but rings, 

 parallel rods, eights open and closed, L's, U's and irregular- 

 shaped bodies (fig. 19, a-l). 



In my earlier study of this phenomenon, I supposed the 

 chromosomes to be the equivalents of the long, coiled segments 

 first formed, and with such an hypothesis the whole series of 

 events following longitudinal fission was inexplicable. But after 

 again considering not only such stages as those represented in 

 figs. 10-17, but every transitional form connecting them, I am 

 convinced that this assumption was incorrect and that each seg- 

 ment consists, rather, of two distinct chromosomes standing 

 side by side, each half of the double chromosome represent- 

 ing two sister-segments which were formed by the earlier longi- 

 tudinal fission but have now fused. If such be the origin of 

 these chromosomes, and I no longer have any hesitancy in 

 affirming that they have thus arisen, the phases following the 



Proc. Wash. Acad. Sci., July, 1904. 



