Sept., i9ii.l YASUI:— ON THE BEHAVIOR OF CHROMOSOMES 157 



1. The chromosome number of the somatic nucleus. According 

 to Tahara (1915), the haploid number in P. oricntalc is 21, and 

 that in P. somniferum 11 which were confirmed in my own observa- 

 tions. The somatic or diploid number of chromosomes in my F^ 

 plant was found to be 32 (PI. Ill, Fig. 1) just the sum of tha haploid 

 numbers in the o^ and -?- parent plants. 



2. The meiotic division of the pollen mother cell. The synaptic 

 knot seems to consist of many small irregular masses. In the earlier 

 spireme stage, no trace of the doubleness of the thread due to the 

 longitudinal splitting or the parallel approach of two threads could 

 be ascertained. When the segmentation of the thread is complete 

 and the chromosomes are established, the latter is clearly found to 

 be 32 in number (PI. Ill, Fig. 6). 



After the second contraction (PI. Ill, Fig. 7), 22 out of 32 

 chromosomes form 11 bivalent chromosomes, consequently there are 

 11 bivalent and 10 univalent chromosomes in the diakinesis (PI. Ill, 

 Fig. 8). Each bivalent chromosome consists of two equivalent 

 chromosomes united end to end, not side by side. 



When the nuclear membrane disappears, the bivalent chromosomes 

 mierate toward the center of the nuclear mass, where they form the 

 equatorial plate, and receive a part of spindle fibres (PI. Ill, Fig. 

 10, 11). At that stage, all the chromosomes show longitudinal splits. 



All the eleven halves of the bivalent chromosmes generally go toge- 

 ther to the anaphase ; but frequently seven of them are already in the 

 anaphase, while the remaining four are found still in the metaphase 

 (PI. Ill, Fig. 12). After halves of each bivalent chromosomes left 

 the equator for the opposite poles, the univalent chromosomes now 

 taking the place of the latter, entering the equatorial region of the 

 spindle, form a secondary equatorial plate (PI. Ill, Fig. 13). In this 

 position, they split longitudinally and their halves follow the preceding 

 chromosomes for the opposite poles. Thus in the hybrid F, plant, 

 all chromosomes do not always pass the metaphase simultaniously, 

 but sometimes in two or even three successions. 



The part played by the univalent chromosomes in the formation 

 of the daughter nuclei is various : (1) Some of them situated too far 

 from the center do not enter the spindle, the result being their 

 exclusion from the nucleus, when the nuclear membrane is established ; 

 (2) univalent chromosomes, though entering the spindle at the equator 

 and move toward the poles, break their step, so that while some of 

 them are already in the telophase, others are left behind and thus 



