﻿66 HaNCE: Pollen DEVELOPMENT IN ZeBRINA PENDULA 



others. It is safe then, I think, to conclude that no odd chromo- 

 somes occur in Zebrina pendula and that any chromosome may act 

 in a manner not unlike that of the well-known accessory chromo- 



Anaphase. — Judging from the size and position (i. e., at one 

 end of the cell) of the chromosomes, Figs. 14, 18, and 19 are 

 anaphases. The paired arrangement of the chromosomes is still 

 apparent and it is evident that the pairs are just one half the 

 size of those in the mother cells. They have resulted from a 

 transverse division of the chromosome pairs and in Fig. 10 the 

 early indication of a transverse split is shown at A and B. Owing 

 to the size of the cells it is practically impossible to find an uncut 

 cell showing both ends of the anaphase in the same section, but 

 Fig. 20 is a drawing carefully built up from two sections. The 

 dark dotted line in the figure merely indicates the line of separation 

 between the two groups of chromosomes and does not appear on 

 the slide. 27 chromosomes appear at each pole and though the 

 paired arrangement is not so apparent as in the preceding figures, 



The dividing wall formed after the heterotypic division is 

 pictured in Figs. 21-23. In the first two drawings chromatic 

 fragments are figured which trail after the other chromosomes 

 and which probably will fail to reach the reforming nucleus in 

 time to be included. Two entire chromosomes have been omitted 

 from the nucleus in Fig. 23. 



Homotypic Division 

 No dividing cells of this stage which permitted a count of the 

 chromosomes have been found. The period of rest between the 

 heterotypic and homotypic divisions is not very long. As in the 

 case of the previous division, chromosomes may fail to reach the 

 nuclei of the tetrads as shown in Fig. 25. Fig. 26 is a drawing of 

 a complete tetrad. 



The Pollen Cell 

 Each member of the tetrad undergoes enormous growth result- 

 ing in the pollen grains pictured in Figs. 29 and 30. The nucleus 

 may divide once but no later stage has ever been observed. 



