1917] Boeck: Mitosis in Giardia microti 15 



seems, however, a tendency for them to return later to approximately 

 the same position as that which they occupied in the split spireme 

 when first formed. The uppermost chromosome of each spireme band 

 comes to lie side by side with its mate of the opposite strand (pi. 1, fig. 

 6; text-fig. I). Lower chromosomes continue this pseudosynaptic side- 

 by-side pairing. After this pairing a rather intimate lateral fusion 

 of these pairs takes place. This is a fusion on a plane identical, or 

 at least parallel, with that of their original separation. What is prob- 

 ably the beginning of this pairing and fusion of chromosomes number 

 1 is seen in the uppermost end of the left nucleus in figure 6 (plate 1), 

 while the same chromosomes in the right nucleus also appear to be 

 approaching each other. The end result of this fusion is four chroma- 

 tinic masses, each composed of two chromosomes which have previously 

 split and then fused (pi. 1, fig. 7; text-figs. J, K). Here again there 

 is evidence that each one of these masses is so situated that its long 

 axis is still nearly the same as that of the nucleus and approximately 

 that of the whole cytoplasmic body. This is also true for plane of 

 fusion, which is parallel to the plane which involves the long axis 

 (pi. 1, fig. 7; text-fig. J). 



The spindle is now formed from the linin of the nucleus. Just 

 previous to spindle formation the linin is a central mass upon which 

 the four chromatinic masses are situated (pi. 1, fig. 7 ; text-figs. J. K). 

 Often it seems to be shaping itself into a spindle-like structure 

 approaching prematurely the structure of the later spindle (pi. 1, 

 fig. 7, left nucleus). The spindle may partially or entirely fill the 

 nucleus (pi. 1, figs. 8, 9). The condition in which the spindle partially 

 fills the nucleus is probably due to plasmolysis at the time of the fixa- 

 tion of the material. 



With spindle formation, these four chromatinic masses come down 

 into the equatorial plate (pi. 1, fig. 8; text-fig. L). On the spindle 

 they appear to be elongated and to be getting ready for tranverse divi- 

 sion in the metaphase. Because of the position of these chromatinic 

 masses on the spindle it is probable that when the chromosomes pre- 

 viously fused side by side (pi. 1, fig. 7) they next began to pull apart 

 at one end and this proceeded more and more until finally these 

 chromosomal members of each chromatinic mass came to an end-to-end 

 position, though still fused in that region and are shown in this stage 

 (pi. 1, fig. 8) in the equatorial plate on the spindle ready for final 

 separation at the close of the metaphase. 



This pairing of chromosomes and their subsequent fusion might 



