DETAILS OF NUCLEAR DIVISION 225 



tative cells of root and stem tips, cambium ring, etc.; but if we 

 count each member of the pair a separate chromosome, as we 

 assumed to be the case in Osmunda, and as the earlier stages 

 seem to justify us in doing, one member perhaps of paternal and 

 the other of maternal origin, then there are just as many 

 chromosomes here as in the vegetative divisions. Each member 

 of the pair now undergoes a longitudinal division (Fig. 129, E), 

 but this is soon lost sight of, due to the shortening and thickening 



B 



i^ '-O 



Fig. 129. — ^Processes of fusion of double nuclear thread followed by their separation, 

 and longitudinal splitting of each. A , double threads before their union, showing their 

 component ids at x; B, double threads in process of uniting, fusion completed in the two 

 upper figures, where pairs of ids have fused as at i; C, completion of fusion with the 

 pairs of ids united as at i in B; D, 1 and 2, two pieces such as the one shown in C, with 

 their fused threads again separating; at i, a and 6, we see the two parts that were fused in 

 C; E, a and 6 correspond to a and 6 of 1 in D: a and b are much thicker in E than in D, and 

 a and 6 in £ have each split longitudinally; o will be drawn to one pole and b to the other, 

 and as they start their longitudinal halves will spread to form Vs as shown in 11 of Pig. 

 128. (After C. E. Allen.) 



of the pair (Fig. 128, 9). In this condition the several pairs 

 become lined up at the equator (Fig. 128, 10). 



In the metaphase the members of each pair begin to separate, 

 the one from the other (Fig. 128, 10, and Fig. 130, B, r and s), 

 and before they are drawn toward opposite poles in the anaphase 

 the longitudinal division in each member again becomes ap- 

 parent by the actual separation of the halves at their equatorial 

 ends (Fig. 128, 11, and Fig. 130, B). 



It will appear later on that there are good reasons for the as- 

 sumption we have been making, that when one of these longitudi- 

 nally split chromosomes now traveling to one pole is of paternal 

 origin, its former mate now traveling to the opposite pole is of 

 of maternal origin. 



At the poles the chromosomes form the nuclei of the two mother 

 cells of the microspores (Fig. 128, 12), and if we count each 

 IS 



