222 



KIYOSHI M.\SUI. 



bivalent claromosomes are formed by the coujugation of the homologous ones 

 (Figs. 9-13). 



The side view of the metaphase plate of the first division shows Hhat 

 twenty bivalent chromosomes are arranged in the equatorial plate with their 

 short axis parallel to the latter (Figs. 43-45). Immediately they begin to 

 divide at their ends where the spindle fibers attach (Figs. 46, 47). "WTaother 

 this dividing line of the chromosome represents the conjugated plane of the 

 univalent chromosomes or whether it is simply the longitudinal splitting which 

 is comparable to that of the somatic chromosome, is difficult to determine 

 positively. But judging fi'om the method of synapsis and from the ring 

 formation, this lino of division seems to be equivalent to the conjugated plane 

 of the univalent chromosomes. If it is s >, then the division must be looked 

 upon as a reducing one. 



When every chromosome is simultaneously divided into two portions U- 

 or V-shaped ones are not produced, but they simply assume the short rod- 

 shape (Fig. 51). In well stained preparations previously treated with a 

 modification of H.\xce's method, two special chromosomes with dfflörent 

 behavior can usually be seen at this period. These may be the sex-chromosomes 

 and consist of a large X- and a small Y-chromosome. In the metaphase 

 these XY-chromosomes are united tj form a bivalent one, appearing as an 

 ordinany bivalent chromos:)me. Together with the division of the ordinary 

 chromosomes the bivalent sex-chromosome usually separates iutj its com- 

 ponents, a large X and a small Y-chromosome, these are usually seen at the 

 beginning of the anaphase, but they can often be seen at the lato metaphase 

 as well (Fig 4G-51). In the metaphase these chromosomes are sometimes 

 seen near the respective poles, while the ordinary ones still remain in the 

 equatorial plate (Fig. 48). This shows that the division or the separation of 

 the bivalent sex-chromosome precedes that of the ordinary ones and its com- 

 ponents X and Y pass to the resjwctive poles in advance of the latter. 



"VMien the chromosomes arrive at the respective poles they ai-e usually 

 separated and so it is not difficult to make out their individualities. In a 

 good polar view of the anaphase wc can distinguish two kimls of giviups of 

 chromosomes, the one containing tlie large X-chromos ome and the otlior the 

 smiill Y (Fig. 13). If we now pifk out all Iho chromosomes and arrange 



